Updated to RSpec 3.0 with :should syntax

It's pretty easy. :-)

Also removed unnecessary MiniTest gem (built into Ruby now) and resolved some deprecation warnings.

```
Finished tests in 0.634248s, 165.5504 tests/s, 105.6369 assertions/s.
105 tests, 67 assertions, 0 failures, 0 errors, 0 skips
```