tox: bump httpretty version to 0.8.14
tox: bump httpretty version to 0.8.14

Version 0.8.10 doesn't work with Python 3.5

Collecting httpretty<=0.8.10
  Downloading httpretty-0.8.10.tar.gz (41kB)
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-build-arwau336/httpretty/setup.py", line 86, in
<module>
        version=read_version(),
      File "/tmp/pip-build-arwau336/httpretty/setup.py", line 46, in
read_version
        finder.visit(ast.parse(local_file('httpretty', '__init__.py')))
      File "/tmp/pip-build-arwau336/httpretty/setup.py", line 78, in
<lambda>
        open(os.path.join(os.path.dirname(__file__), *f)).read()
      File "/test/.tox/py35/lib64/python3.5/encodings/ascii.py", line
26, in decode
        return codecs.ascii_decode(input, self.errors)[0]
    UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position
133: ordinal not in range(128)

State/stacks,cfstacks/stacks