Ignore the Python binary from virtualenv on Mac

This is the location of the Python interpreter that's installed to the root of a virtualenv that's created at the root of a git repository.

<pre>$ virtualenv -v test
....
Creating test/bin
New python executable in test/bin/python
Changed mode of test/bin/python to 0755
Symlinking test/.Python
....
$ file Test/.Python
Test/.Python: Mach-O universal binary with 2 architectures
Test/.Python (for architecture i386):	Mach-O dynamically linked shared library i386
Test/.Python (for architecture x86_64):	Mach-O 64-bit dynamically linked shared library x86_64
</pre>