make development requirements open ended
-cryptography==1.0.1
-bumpversion==0.5.3
-wheel==0.23.0
-watchdog==0.8.3
-flake8==2.4.1
-tox==2.1.1
-coverage==4.0
-Sphinx==1.4
-PyYAML==3.11
-alabaster==0.7.7
-sphinx_rtd_theme==0.1.9
-Pygments==2.1.3
-Babel==2.3.4
+cryptography >= 1.0.1
+bumpversion >= 0.5.3
+wheel >= 0.23.0
+watchdog >= 0.8.3
+flake8 >= 2.4.1
+tox >= 2.1.1
+coverage >= 4.0
+Sphinx >= 1.4
+PyYAML >= 3.11
+alabaster >= 0.7.7
+sphinx_rtd_theme >= 0.1.9
+Pygments >= 2.1.3
+Babel >= 2.3.4
