DS-2133 : Update Travis to install Mirage2 prerequisites and build Mirage2

Updated PR (to replace PR #636). This one provides a couple of extra fixes around Unit Testing which seemingly help avoid Travis CI "killed" messages. These fixes include:

* First, just enabling Mirage2 build process for Travis CI. I chose to preinstall Mirage2 prerequisites via our .travis.yml (in the `before_install` settings) to speed up the Mirage 2 build. See https://github.com/tdonohue/DSpace/commit/a0f44f618244964df5caf7402452fd0511d153da
* Improve the memory management of Unit Tests. Namely, ensure any Context objects are fully closed and set to null, to try to decrease memory usage of individual Unit Tests. See https://github.com/tdonohue/DSpace/commit/d4d63083bbca66dcf506ff5dc437f109680ab013
* Minor POM reorganization which *disables* all Unit Testing related Maven Plugins by default (i.e. they are only enabled when unit testing is being performed). This is needed to enable the next Travis CI tweak.. See https://github.com/tdonohue/DSpace/commit/bd98997dd69996139a051e5d0fcc045ff07af12a
* Change Travis CI settings to run `mvn install -P !dspace` PRIOR to the actual build & test. The idea is to split up the downloading of dependencies from the build & test, to hopefully help keep our memory usage lower. Therefore, this setting ensures most DSpace Maven dependencies are downloaded, but it *skips* building the 'dspace' module (so DSpace isn't actually packaged up), which speeds up this initial build. The "-P !dspace" setting required the POM reorg mentioned above. See https://github.com/tdonohue/DSpace/commit/8b53258b5ee8ba7ddf76e505f5b65aa6b6dfa57f

This all seems to be working much better on my own Travis CI builds. Let's see what Travis says about this new PR though.