Change travis cache to cache .m2/repository, not all of .m2
Change travis cache to cache .m2/repository, not all of .m2

This should (?) allow settings.xml changes to propagate down to our build, but still cache copied versions of dependency jars.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=310633203

google/truth,google/truth,google/truth