Changes tar ball and repo to mirror of Redmine.

Redmine community has following github repository which is indicated in [Download page](http://www.redmine.org/projects/redmine/wiki/Download).
https://github.com/redmine/redmine

I think the above repository is more appropriate than the current one in .travis-init.sh.
Note: http://www.redmine.org/boards/1/topics/42747