Convert binary Popen output into str

I tried install pygraphviz with Python 3.4 on OS X and eventually I got error in distutils.

      File "/usr/local/Cellar/python3/3.4.1_1/Frameworks/Python.framework/Versions/3.4/lib/python3.4/distutils/unixccompiler.py", line 205, in library_dir_option
        return "-L" + dir
    TypeError: Can't convert 'bytes' object to str implicitly

This fix helps to install it correctly (works both Python 3.4.1 and Python 2.7.6 )