Fix Code Coverage

`Rcov` does not work on Ruby 1.9.x. Since the .rvmrc file loads ruby-1.9.3, issues can be expected as well as inaccurate tests. As recommend by `rcov`, coverage for ruby-1.9.3 is recommend to use `SimpleCov`.

`rake spec` will automatically generate the code coverage file. As of this PR, the coverage is 90.5%.