Create a new rails application with a specific version

instructions.txt

# make directory for new rails app
mkdir app
cd app

# specify ruby version 
echo 2.3.1 > .ruby-version

# initialize bundler (creates Gemfile)
bundler init

# specify rails version in Gemfile
gem "rails", "4.2.5"

# create rails app using specified version, and overwrite current Gemfile
bundle exec rails new . --force