use simplecov version 0.7.1

according to a note on the simplecov's project page one should even
force the usage of version 0.7.1

see:
https://github.com/colszowka/simplecov#important-notice-there-is-currently-a-bug-that-affects-exit-code-handling-on-the-08-line-of-simplecov-see-281-please-use-gem-simplecov--071-until-this-is-resolved