Get travis to install wheel version of pyenchant

Wheel package contains dependencies
But I don't think it installs the package, it simply creates the wheel for later installation. If you install it into virtualenv (this is what should be happening on travis), buildout should be able to figure out, that it is installed and nothing to do. And actually I just tried to exactly that locally and it worked.

* `./bin/buildout` failed due to missing C library
* `pip install -U pip`
* `pip install pyenchant`
* `./bin/buildout` worked