fix travis config for seamless PIL installation

[timdiller]

Fixes the travis.yaml file to allow an unverified installation of PIL

[enbuilder]

Can one of the admins verify this patch?

[jonathanrocher]

@timdiller could you provide details on the motivation for this?

[timdiller]

My PR #162 caused the Travis CI to error https://travis-ci.org/enthought/chaco/builds/19114294. The cause of the error was failure to load PIL properly:

Downloading/unpacking PIL (from enable)
  Could not find any downloads that satisfy the requirement PIL (from enable)
  Some externally hosted files were ignored (use --allow-external PIL to allow).
Cleaning up...
No distributions at all found for PIL (from enable)
Storing debug log for failure in /home/travis/.pip/pip.log
The command "pip install git+http://github.com/enthought/enable.git#egg=enable" failed and exited with 1 during install.
Your build has been stopped.

This is related to @itziakos comment here

The fix in the .travis.yml forces acceptance of the PIL install, which is necessary for enable to load properly.

I don't know for sure why this is a problem now and it wasn't recently, but I think it may be due to an update in pip.

[timdiller]

Please forgive the terribly named PR...

[itziakos]

This is related to @itziakos comment here

Just to clarify:

The issue discussed in the mailing list is not related with the fix in this PR. The Fix is this PR is necessary because the new version of pip does not install packages that are external to the pypi repositories. Since PIL is external and enaml needs PIL, to install we have to tell pip to specifically allow PIL to be installed.

[jonathanrocher]

Thanks for catching this. Merging.