setup.cfg: Ignore file test-requirements.txt

`test-requirements.txt` has been added to
addopts --ignore flag which ensures it doesn't
get picked up when using `py.test`

Fixes coala#71

setup.cfg

@@ -9,7 +9,7 @@ python_files = *Test.py
 python_classes = *Test
 python_functions = *_test
 timeout = 35
-addopts = --color=yes
+addopts = --color=yes --ignore=test-requirements.txt
 env =
     PYTHONHASHSEED=0
 # PYTHONHASHSEED=0 is required to use same hashes in pytests-xdist's workers