Updated Simplecov to 0.9.1, as there is a bug in 0.8.x

According to the Simplecov Github repo: 

>Important Notice: There is a bug that affects exit code handling on the 0.8 line of SimpleCov, see #281.  Please use versions ~> 0.7.1 or ~> 0.9.0 to avoid this.

https://github.com/colszowka/simplecov

So Simplecov is updated to 0.9.1.