Pass allow-external to pip install to allow PIL installation

With recent changes to pip 1.5, insecure external URLs need to extra flag to
be installed. This commit fixes the python_deps.sh script by setting this
flag. Alternatively, the dependency on PIL could be replaced by a dependency
on Pillow which is a maintained fork of PIL.
Revert "Pass allow-external to pip install to allow PIL installation"

This reverts commit e8d423f97acf6aa8e17489c7eb2b068909328770.
Do not install Python dependencies in a separate virtualenvironment

Since using Homebrew Python, all dependencies should be installed under
/usr/local.
Merge pull request #1946 from sbesson/rebased/develop/homebrew_pip_pil

Install Python dependencies using Homebrew Python (rebased onto develop)