Use require instead of load with test/unit/version

  Other gems also require test/unit/version. When this gem loads the
same file it over writes the constant and causes a warning.
Merge pull request #140 from tmiller/master

Use `require` instead of `load` with `test/unit/version`.

Other gems also require `test/unit/version`. When mocha loads the
same file it over writes the constant and causes a warning.

I tried to reproduce the original problem [1] with more recent
versions of the test-unit and mocha gems, but I ran into a
different exception. This exception was fixed by adding a
`gem "test-unit"` statement before the `require "test/unit"`
statement.

[1] http://floehopper.lighthouseapp.com/projects/22289-mocha/tickets/50#ticket-50-13