tests: bower cache and travis instance config

* Adds caching of bower components.

* Adds Travis invenio.cfg to prevent multiple calls to inveniomanage.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
tests: travis timing

* Adds timing information to Travis build to investigate which
  commands that can be optimized.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>