Use Appraisal for testing multiple Rails versions.

This makes maintaining and update the multiple gemfiles a bit easier,
and the rake task also works under JRuby (the existing `rake
compatibility` task doesn't for some reason).
Merge branch 'appraisal' of https://github.com/GUI/activerecord-postgis-adapter into GUI-appraisal

Conflicts:
	.travis.yml
Merge branch 'GUI-appraisal'