DS-2075 : Decrease number of log messages for Travis CI & our sanity

Related to https://jira.duraspace.org/browse/DS-2075

Two included tweaks:

1. Tiny POM tweak tells maven-surefire-plugin to no longer dump all the unit testing logs onto stdout, instead they are written to the appropriate `target/surefire-reports/*-output.txt` file. This keeps the Unit testing details on STDOUT much more at a summary level, so you'll still see messages like this when running Maven:

        Tests run: 24, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 20.866 sec - in org.dspace.authenticate.IPMatcherTest
But, all the extra INFO messages, etc are sent into a log file under `target/surefire-reports/`

2. Tiny travis.yaml tweak to tell Travis CI to cache the contents of the `~/.m2` directory. This should decrease logs and also speed up Travis build time. See: http://docs.travis-ci.com/user/caching/

Both of these should help fix some current issues where Travis CI is killing our builds because our log files grow above 4MB in size (mostly because of these detailed unit test logs being sent to STDOUT).