Fix syntax error in Python 3.3.

“Unicode Raw” strings are no longer supported in Python 3.3, and cause a syntax error. See http://bugs.python.org/issue15096.

I understand that the source distribution of Pystache is not intended to by Python 3 compatible out of the box, but this syntax error seems unnecessary. With this change, the code is functionally equivalent, and the syntax is compatible with all Python versions.