Move testenv requirements into test-requirements

Neaten up the tox.ini by moving testing requirements into their own
test-requirements.txt

Author: jhesketh

test-requirements.txt

@@ -0,0 +1,4 @@
+coverage
+pretend
+pytest
+pytest-capturelog

tox.ini

@@ -3,11 +3,8 @@ envlist = py27,py34,pep8,pip-missing-reqs
 
 [testenv]
 deps =
-    coverage
-    pretend
-    pytest
-    pytest-capturelog
     -r{toxinidir}/requirements.txt
+    -r{toxinidir}/test-requirements.txt
 commands =
     coverage run --source pip_missing_reqs/find_missing_reqs.py \
         -m pytest --strict []
@@ -25,6 +22,8 @@ deps = flake8
 commands = flake8 pip_missing_reqs
 
 [testenv:pip-missing-reqs]
+# Overwrite deps so not to dirty the environment
+deps = -r{toxinidir}/requirements.txt
 commands = python -m pip_missing_reqs.find_missing_reqs \
     --ignore-file=pip_missing_reqs/test* pip_missing_reqs