Backported jaquadro's 1.7.2 fix for PlantMegaPack compatibility

This adds full compatibility for the 1.6.x version of the Plant Mega Pack mod.

*This will break automerge on [my other 1.6.x PR](https://github.com/jaquadro/ForgeMods/pull/4) and if the other PR is applied first, it will break automerge on this PR.*