Travis: never send emails on success

This silences passing notifications for new branches
Switch to minitest for consistent output between Ruby versions

In Ruby 2.0, `test/unit` activates a fancy output style for terminals
that doesn't look well in Travis logs. I could disable it with the
`--jobs-status=normal` test optiont, but that option isn't supported in
Ruby 1.8.7.

Instead of dealing with inconsistencies between test support in
different Ruby versions, I'm simply switching to a fixed version of
Minitest.
Update Cucumber for better Minitest compatibility

Otherwise it hits deprecation warnings because it accesses the
Minitest::Unit module.
Merge branch 'minitest'