[MARVIN-24] Pytest-Cov Version Requirement Update

Pytest-cov requirement update: https://pypi.org/project/pytest-cov/
Travis-ci fail: https://travis-ci.org/apache/incubator-marvin/builds/476350343
Based on this fix: https://github.com/pywbem/pywbem/pull/1370/files
My testing: https://travis-ci.org/Wei-1/incubator-marvin/jobs/476724419
diff --git a/python-toolbox/setup.py b/python-toolbox/setup.py
index fce8991..8e4526d 100644
--- a/python-toolbox/setup.py
+++ b/python-toolbox/setup.py
@@ -88,7 +88,7 @@
     'tox>=2.2.0',
     'mock>=2.0.0',
     'pytest>=2.9.2',
-    'pytest-cov>=1.8.1',
+    'pytest-cov>=1.8.1,<2.6',
     'pytest-watch>=4.1.0',
     'pytest-testmon>=0.8.2',
     'Keras>=2.2.0',