Collect the dependencies from the variants and not the buildTypes

The Android-gradle API changed, this works around it. It's still pretty
bruteforce and I'm also ignoring variantDependency.getLocalDependency().

Untested fix for novoda/robolectric-plugin#5
Merge pull request #7 from BurntBrunch/fix/tools-0.5

Collect the dependencies from the variants and not the buildTypes