Don't require Bundler at runtime.

Note that this reverts commits 1c5979d and 060fa5a, requiring you to
"bundle exec" if you're running "bin/bosh-bootstrap" from within
the gem source directory.

Fixes cloudfoundry-community/bosh-bootstrap#218.
Fixes cloudfoundry-community/bosh-bootstrap#219.
Merge pull request #220 from mdub/no_bundler_at_runtime

Don't require Bundler at runtime.