Updated CONTRIBUTING to indicate that going forward we require the use of Prophecy for test doubles.
Updated CONTRIBUTING to indicate that going forward we require the use
of Prophecy for test doubles.

Addresses #983

localheinz/opencfp,PHPBenelux/opencfp,GrUSP/opencfp,GrUSP/opencfp,mdwheele/opencfp,localheinz/opencfp,mdwheele/opencfp,mdwheele/opencfp,mdwheele/opencfp,PHPBenelux/opencfp,GrUSP/opencfp,GrUSP/opencfp,opencfp/opencfp,opencfp/opencfp,PHPBenelux/opencfp,localheinz/opencfp,opencfp/opencfp,PHPBenelux/opencfp,localheinz/opencfp