Remove code made obsolete by Bundler dependency

Since 2cdf739baa1a7c1a0aa86649c1b0ca902f9f4bad started always loading
bundler/setup during tests, we can get rid of some manual load path
setup and dependency checking. We should also explicitly add bundler as
a development dependency.
move development dependencies to Gemfile, & use Bundler.require().
use false literal in place of string
Merge pull request #1159 from pry/bundler

Remove code made obsolete by Bundler dependency