Enable Java 1.8 in the open source build and bump the guava version to 21.0
Enable Java 1.8 in the open source build and bump the guava version to 21.0

For the near future we will likely not be using any java8 features in the
actual compiler, due to limitations in various internal google products.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=166202602

yext/closure-templates,google/closure-templates,Medium/closure-templates,Medium/closure-templates,google/closure-templates,google/closure-templates,Medium/closure-templates,yext/closure-templates,Medium/closure-templates,yext/closure-templates,google/closure-templates,google/closure-templates,yext/closure-templates,Medium/closure-templates