tox: turn on 'usedevelop'

This should fix the coverage data's filenames: previously they were like
".tox/coverage/lib/python2.7/site-packages/wormhole/foo.py", now they
should be "src/wormhole/foo.py".

tox.ini

@@ -20,6 +20,7 @@ skip_missing_interpreters = True
 deps =
     pyflakes >= 1.2.3
     mock
+usedevelop=true
 install_command = pip install {packages}
 commands =
     pyflakes setup.py src
@@ -39,6 +40,7 @@ deps =
     pyflakes >= 1.2.3
     mock
     coverage
+usedevelop=true
 install_command = pip install {packages}
 commands =
     pyflakes setup.py src