Gemfile

source 'https://rubygems.org'

gem 'activerecord', '4.2.0'
gem 'sqlite3'
gem 'bcrypt', '~> 3.1.7'

Gemfile.lock

GEM
  remote: https://rubygems.org/
  specs:
    activemodel (4.2.0)
      activesupport (= 4.2.0)
      builder (~> 3.1)
    activerecord (4.2.0)
      activemodel (= 4.2.0)
      activesupport (= 4.2.0)
      arel (~> 6.0)
    activesupport (4.2.0)
      i18n (~> 0.7)
      json (~> 1.7, >= 1.7.7)
      minitest (~> 5.1)
      thread_safe (~> 0.3, >= 0.3.4)
      tzinfo (~> 1.1)
    arel (6.0.0)
    bcrypt (3.1.9)
    builder (3.2.2)
    i18n (0.7.0)
    json (1.8.1)
    minitest (5.5.0)
    sqlite3 (1.3.10)
    thread_safe (0.3.4)
    tzinfo (1.2.2)
      thread_safe (~> 0.1)

PLATFORMS
  ruby

DEPENDENCIES
  activerecord (= 4.2.0)
  bcrypt (~> 3.1.7)
  sqlite3

minitest.rb

require 'rubygems'
require 'bundler/setup'
require 'bcrypt'
require 'psych'

BCrypt::Password.create("my password").to_yaml

test.rb

require 'rubygems'
require 'bundler/setup'
Bundler.require(:default)

require 'active_record'

ActiveRecord::Base.establish_connection(
  adapter:  "sqlite3",
  database: ":memory:")

ActiveRecord::Migration.create_table :users do |t|
  t.string :password_digest
end

class User < ActiveRecord::Base
  has_secure_password
end

user = User.new
user.password = 's3cr3t'
user.to_yaml

~/work/SR/bcryptest2 » rbenv shell 2.1.5                                                                        be9@MBPbe9
------------------------------------------------------------
~/work/SR/bcryptest2 » ruby test.rb                                                                             be9@MBPbe9
-- create_table(:users)
   -> 0.0024s
------------------------------------------------------------
~/work/SR/bcryptest2 » rbenv shell 2.2.0                                                                        be9@MBPbe9
------------------------------------------------------------
~/work/SR/bcryptest2 » ruby test.rb                                                                             be9@MBPbe9
-- create_table(:users)
   -> 0.0022s
/usr/local/var/rbenv/versions/2.2.0/lib/ruby/gems/2.2.0/gems/bcrypt-3.1.9/lib/bcrypt/engine.rb:54:in `hash_secret': invalid salt (BCrypt::Errors::InvalidSalt)
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/gems/2.2.0/gems/bcrypt-3.1.9/lib/bcrypt/password.rb:66:in `=='
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:315:in `visit_String'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:152:in `accept'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:560:in `block in dump_ivars'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:558:in `each'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:558:in `dump_ivars'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:345:in `visit_String'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:152:in `accept'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:377:in `block in visit_Hash'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:375:in `each'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:375:in `visit_Hash'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:152:in `accept'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:547:in `block in emit_coder'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:545:in `each'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:545:in `emit_coder'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:530:in `dump_coder'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:150:in `accept'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/visitors/yaml_tree.rb:116:in `push'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych.rb:409:in `dump'
    from /usr/local/var/rbenv/versions/2.2.0/lib/ruby/2.2.0/psych/core_ext.rb:14:in `psych_to_yaml'
    from test.rb:21:in `<main>'