Imporve tox and Travis configuration

7cb60437f352ccbd417a4e98abb9f233e3803525 and 6b750f75e3e4f48dcc630fae2ae81ea9bf7fc0e0 are minor fixes for testing.

348edf266850f5d2a051aad910afba29aa35527f improves tox and Travis configuration:
* Both use requirements-dev.pip.
* Now tox tests with latest Flask and {py26,py27,pypy} .
* Now Travis tests with Flask {0.8,0.9.0.10} and {py26,py27,pypy}.

Here is the Travis build log: https://travis-ci.org/papaeye/flask-assets/builds/8595522.