bugfix: old mock module does not mock open properly

.travis.yml

@@ -51,7 +51,7 @@ install:
     for pv in "2.7" "3.4" "3.5"; do
         source ~/virtualenv/python${pv}/bin/activate
         # explicitly install all needed python modules to cache them
-        for p in '-r requirements.txt' 'behave codacy-coverage coverage coveralls flake8 mock pytest-cov pytest'; do
+        for p in '-r requirements.txt' 'behave codacy-coverage coverage coveralls flake8 mock>=2.0.0 pytest-cov pytest'; do
             pip install $p
         done
     done

setup.py

@@ -147,7 +147,7 @@ def setup_package():
         install_requires=install_reqs,
         setup_requires=['flake8'],
         cmdclass=cmdclass,
-        tests_require=['mock', 'pytest-cov', 'pytest'],
+        tests_require=['mock>=2.0.0', 'pytest-cov', 'pytest'],
         command_options=command_options,
         entry_points={'console_scripts': CONSOLE_SCRIPTS},
     )