Fix Java debugger GCE/MVM E2E tests. The tests got broken today because there was a new Guava release (v21.0-rc1) today which was built with Java8, but we assume all dependencies of JniProxyCodeGen tool are Java7.
Fix Java debugger GCE/MVM E2E tests. The tests got broken today because there
was a new Guava release (v21.0-rc1) today which was built with Java8, but we
assume all dependencies of JniProxyCodeGen tool are Java7.

This CL does two things:

1. It pins the dependency to Guava version 20.0, which fixes the breakage.
2. It adds a maven plugin that fails the build if any of the dependencies are
built with a JDK version higher than 1.7, which will (hopefully) give us a
more meaningful build error in the future.

We don't have to pin ourselves to Java7 in the GCE world, but I think it is good to fully support Java7 users for now.

http://sponge/891e20b1-e877-4a51-8b7d-43940bca2709

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GoogleCloudPlatform/cloud-debug-java,GoogleCloudPlatform/cloud-debug-java,GoogleCloudPlatform/cloud-debug-java,GoogleCloudPlatform/cloud-debug-java