brew-test-bot: check for hg in requirements
the_platinum_searcher 1.7.1

the_platinum_searcher (pt) now uses Godeps to vendor its dependencies.
the_platinum_searcher: correct system call syntax
the_platinum_searcher: use Go dependency helper

Use Homebrew's Go dependency helper to install all dependencies
specified in the_platinum_searcher's Godeps file.
the_platinum_searcher: use hg for dependency
the_platinum_searcher: update 1.7.1 bottle.