Revert "Attempt to fix ENOTEMPTY on bower install on Travis"

This did not resolve the issue.

https://www.openproject.org/work_packages/5645

This reverts commit aab77f6c1921d12b4476eaffa124ed60285a01ce.
Configure Travis to use @knalli's Bower fork

Fork includes a fix for possible race condition when installing/
resolving dependencies.

https://github.com/bower/bower/issues/933
https://www.openproject.org/work_packages/5645

Signed-off-by: Alex Coles <alex@alexbcoles.com>