Only require power_assert if version > 1.8.7

The power_assert gem uses hash syntax that is not valid in Ruby version 1.8.7

I've made two changes:

1. Only require the power_assert gem if Ruby version is > 1.8.7
2. Stick to hash syntax valid in 1.8.7, as test-unit should support back to this version. I realize that line 135 won't even run in version 1.8.7 because PowerAssert wouldn't be defined, but syntax should be consistent, until the test-unit gem says it doesn't support 1.8.7.