Revert "Since rspec 3.0 be_true is renamed to be_truthy (works the same)"

This reverts commit 3ad5d86a67426e14efabf2779826a3b1c07511c5.
Revert "added the possibility to use the -alpha option from imagemagick, and added tests for it"

This reverts commit 0cf31d50893329fb0d3f41a89b4fc28be9cd0612.