Add minitest gem to fix test runtime error with ruby 2.0

    undefined method `_run_suite' for class `Test::Unit::Runner
Travis CI: add configuration for Rails 4.0; try fix for minitest in edge
Revert "Add minitest gem to fix test runtime error with ruby 2.0"

This reverts commit 08584e571e9d5f8e40f2b73d1bc99b60878eb314.
Switch to pre 5.0.0 minitest
Merge remote-tracking branch 'upstream/master'