Instrument the Buildpack

https://github.com/heroku/heroku-buildpack-ruby/blob/master/lib/language_pack/instrument.rb#L10

```
measure.ruby.slug_vendor_base.start=2013-07-12T21:15:07.731466+00:00
measure.ruby.slug_vendor_base.end=2013-07-12T21:15:07.799216+00:00
measure.ruby.slug_vendor_base.duration=0.06752991676330566
measure.ruby.slug_vendor_base.level=5
measure.ruby.slug_vendor_base.build_id=0e00dc67-2d7a-4818-9d06-c244ea6ffcda
measure.ruby.slug_vendor_base.buildpack_version=v67