Output the dependency versions for this build of Pages.

This allows easy access to the Gemfile bundler directives, which may
be easily used to tweak versions for upgrades, etc.
Add executable.
Test the executable
Print directly in executable. h/t @ixti
Create two subcommands: 'versions', 'br'.

'versions' prints out the gem declarations for the individual dependencies.
'br' prints out the gem declaration for this gem, based on its repo and the branch name provided.
Update cibuild for new subcommands.
Merge pull request #73 from github/gimme-gem-directives

Output the dependency versions for this build of Pages.