bump mocha version to ~> 1.1
Bump to 1.6.0

Support for Mocha ~> 1.1.
tests: switch to minitest

Ruby 1.9+ uses Minitest as the backend for Test::Unit. As of Minitest 5,
the shim has broken some compatibility with Test::Unit::TestCase in some
scenarios.

Adjust the test suite to support Minitest 5's syntax.

The performance tests do not easily port over, so remove them.