Create facade for switching assertion libraries

This is in preparation of requiring different testing libraries based on
the rails version.
Require minitest for rails >= 4.1.0
Consistently use correct Minitest constant
Update cucumber development dependency
Just assign the constant for the module we want directly
Also includes `refute` assertions
Minitest is the 'future' ... let's just use that name everywhere
#all is the preferred way to get a relation in Rails 4
Makes it clear that all these things are private/internal

[ci skip]
Changelog for #772
Merge branch 'switch-from-test-unit-to-minitest'