Specify version of minitest. Tests do not run on 2.0 otherwise.

I wasn't able to run unit tests on Ruby 2.0. I came across http://stackoverflow.com/questions/17461245/what-does-this-minitestunittestcase-warning-mean , suggesting that the error was due to using minitest 5.0 when the code was written for a previous version.

Without this change, no tests at all get run on Ruby 2.0.