decode text files

For whatever reason, in our use of sphinx-to-github, the addon throws this error:

    File "/Users/deus/code/cloudant/api-reference/venv/lib/python2.7/site-packages/sphinxtogithub/sphinxtogithub.py", line 40, in process
    return text.replace( self.from_, self.to )
    UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in position 2091: ordinal not in range(128)

To fix it, I made sure to decode each text file before replacing the directory names in them.

Tests still pass.