adjust dev reqs

requirements-dev.txt

-sphinx==1.6
-sphinx-rtd-theme==0
-mock==2
-coverage==4
-pytest==3
-pytest-cov==2
-pytest-runner==3
+sphinx==1.6.*
+sphinx_rtd_theme==0.*
+mock==2.*
+coverage==4.*
+pytest==3.*
+pytest-cov==2.*
+pytest-runner>=2.*,<=3.*