Fixes to work with modern webassets and Django 1.6.5

There are two fixes here, afb41b4 is the big one, it's actually only a few lines, but for some reason git sees the whole file as changed, the change is just to add the environment to the context on line 66. It addresses open issue #39 in django_assets.

The second change 266888d allows for running under Django test-runner, which now has DEBUG=False set in the test-environment setup. The result of that is that developers wind up seeing failing test suites where assets reports that the final published location of the combined file isn't present. Alternately, some documentation would be needed to warn users of the gotcha.