Try to avoid triggering an error about coverage when we don't care about it ^^

[shulard]

Hello !

I just encountered a strange issue when executing PHPSpec in a context where there isn't any coverage driver active.

It seems legit to get this error but when I run PHPSpec with the --no-coverage flag it might be interesting to avoid sending this error because we don't care about the coverage in this case…

[FriendsOfPhpSpec\PhpSpec\CodeCoverage\Exception\NoCoverageDriverAvailableException]  
  There is no available coverage driver to be used.

[SebastianBergmann\CodeCoverage\NoCodeCoverageDriverAvailableException]  
  No code coverage driver available