Tremedously speed up testing by only doing coverage reporting on the actual source files

Previously coverage would include the `.tox` directory among others, making everything very slow.

Author: ntninja

tox.ini

@@ -2,7 +2,7 @@
 envlist = py27, py34, py35, py36, py37, pypy3
 
 [testenv]
-commands = py.test --cov=./
+commands = pytest --cov=multiaddr
 
 ; If you want to make tox run the tests with the same versions, create a
 ; requirements.txt with the pinned versions and uncomment the following lines: