METRON-2155 Cache Maven in Travis CI Builds

[nickwallen]

This PR attempts to resolve the recent spat of failed Travis CI builds. These builds seem to fail when attempting to download Maven from the Apache Mirrors.

This PR attempts to cache Maven so it does not need to be repeatedly downloaded from the Apache Mirrors. This would mitigate the issue if there are transient problems with Travis connectivity to the Apache Mirrors or if the Apache Mirrors themselves are having sporadic outages.

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