Rails 4.x / MiniTest 5.x Compatability.

MiniTest::Spec now uses a thread local variable to use with spec expectations. Set this just as MT:S does for AS:TC by using a simple initialize method.

* Removed some 1.8 only code.
* Removed utils to check Rails versions. Moot now.
* Use newer `assert_not` rails assertion vs deprecated/removed `assert_blank`.