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

https://jira.duraspace.org/browse/DS-2133

This change should be pretty self-explanatory. This PR just updates our `.travis.yml` to first install all the Mirage2 prerequisites (based on https://github.com/DSpace/DSpace/tree/master/dspace-xmlui-mirage2) and then ensure Mirage2 is built in our Travis CI test build.

I'll make sure Travis CI is OK with these changes before merging.