Properly test against multiple versions of activesupport

Travis-CI allows you to [test against multiple versions of dependencies](http://about.travis-ci.org/docs/user/languages/ruby/#Testing-against-multiple-versions-of-dependencies-(Ruby-on-Rails%2C-EventMachine%2C-etc\)).

The way this was done in this gem is not the correct way.
As you can see here, The ENV does not make any difference
https://travis-ci.org/cloudsdaleapp/arcane/jobs/9029170
https://travis-ci.org/cloudsdaleapp/arcane/jobs/9029171

This pull request changes the travis build to use multiple gemfiles to test against multiple versions of activesupport