Editorial pass over generated Gemfile

* When run with default options, no repeated blank lines
* Every gem has a comment, perhaps a generic one, but a comment nonetheless
* Most comments used to start with "Use", some with "To use" => made consistent
Merge pull request #10170 from rubys/editorial_pass_over_generated_gemfile

Editorial pass over generated Gemfile
Remove warning

warning: `*' interpreted as argument prefix
application loader refactor and test suite complete rewrite
fixes remaining lowercase "rails"