Attempts to fix #36.

The missing classes were from Guava, so I un-excluded Guava from the dependency declarations—I'm not sure why it was excluded before so I don't know if this is acceptable.

Other than the missing Guava class errors, 4 other tests fail with Gradle 5.6+. I'm a bit over my head in debugging those errors at the moment but this is at least a start toward Gradle 5.6 support.