pin test dependencies to working versions

Author: schlamar

pytest-cov/tox.ini

@@ -7,7 +7,8 @@ setenv =
     PYTHONHASHSEED = random
 deps =
     {env:COV_CORE_DEP:../cov-core}
-    pytest
+    execnet>=1.2.0,<1.3.0
+    pytest>=2.6.4,<2.7.0
     pytest-xdist
     virtualenv
 commands = py.test -v test_pytest_cov.py {posargs}