More memory fixes for Travis CI, related to DS-2133

Since merging #637 (for https://jira.duraspace.org/browse/DS-2133), we still seem to be having some Travis CI issues.

Recent PRs seem to still be having some random "Killed" errors from Travis CI, seemingly related to memory issues (or at least that's what is claimed in the Travis docs here: http://docs.travis-ci.com/user/common-build-problems/#My-build-script-is-killed-without-any-error ) 

This PR seeks to solve that (and after 5 tests with my own Travis it *seems* to work).

* Adds a new "surefire.argLine" Maven option (to our main POM), which lets you specific JVM options to our Unit Tests (i.e. maven-surefire-plugin). By default 'maven-surefire-plugin' forks off a new JVM to run tests, so it ignores any settings in MAVEN_OPTS or JAVA_OPTS
* Tweaks the `.travis.yml` file to use this new `-Dsurefire.argLine` setting, in order to ensure our Unit Tests are getting enough memory on Travis servers.

We'll see what Travis thinks of this PR. As mentioned, it's working for me on my own Travis repo (tdonohue/DSpace), so hopefully it'll work just as well in DSpace/DSpace.