Updated ruby and rubygem version numbers

[zacharysang]

The env vars pointing to the version number for Ruby and RubyGems are outdated in the ruby_rails recipe. The necessary updates are as follows:

• RUBY_MAJOR: 2.3 -> 2.5
• RUBY_VERSION: 2.3.0 -> 2.5.1 (Will also need to update the corresponding checksum)
• RUBYGEMS_VERSION: 2.6.2 -> 2.7.6
• RAILS_VERSION: 4.2.6 -> 5.2.0