quick fix for pip 10 and python 2

[leon0707]

fix #6

[leon0707]

@clemfromspace Hi, would you mind taking a look at the test?

[clemfromspace]

Hey @leon0707, thanks for the PR!

Just looked at the output from travis and found this error:

...
../../../virtualenv/python3.6.3/lib/python3.6/site-packages/twisted/internet/address.py:111: in UNIXAddress
    name = attr.ib(converter=attr.converters.optional(_asFilesystemBytes))
E   TypeError: attrib() got an unexpected keyword argument 'converter'

Looks somewhat similar to mozilla/coverage-crawler#140

Concerning the content of the PR, thanks a lot for the fix concerning pip 10 👍

As for the support of Python 2.*, it may need a lot of changes, since I used the "f" string interpolation, so the package is not currently compatible at all with Python 2 (here and in other places)

[clemfromspace]

@leon0707 I fixed the tests in #8 , please rebase your PR :)