Have tests use Appraisal instead

While working with Active Admin in order to get it working on Rails 4, I decided to spend some time cleaning up the way that the test suite is run. I added in Appraisal from thoughtbot for testing against multiple Gemfiles, and have streamlined the spec_helper.rb and env.rb.

Additionally, I have removed Parallel Tests.

I would love some feedback on what I've done, and if I removed something that you would prefer to keep.

**NOTE:** I have made this pull request to the master branch, but have based my work off of the metasearch->ransack branch. I hope that isn't confusing, but I wanted to be based off of what will be merged in to master soon (based on your comment here: https://github.com/gregbell/active_admin/pull/2120#issuecomment-18799811)