Use this in Bitrise's Ruby Script runner step to increment CURRENT_PROJECT_VERSION.

increment-current-project-version.rb

# Gemfile:
#
# source 'https://rubygems.org'
# 
# gem 'xcodeproj'

require 'xcodeproj'

project_path = 'Your.xcodeproj'
project = Xcodeproj::Project.open(project_path)
target = project.targets.find { |target| target.name == 'TargetName' }

target.build_configurations.each do |config|
  new_version = (config.build_settings['CURRENT_PROJECT_VERSION'].to_i + 1).to_s
  config.build_settings['CURRENT_PROJECT_VERSION'] = new_version
end

project.save()