requirements: pin all versions

Flask and Werkzeug are the worst offenders, but pin everything just for
consistency.

This should prevent docker builds from breaking randomly.

requirements.txt

@@ -1,11 +1,11 @@
-Flask>=0.12
-Werkzeug>=0.14.1
-PyYAML>=3.11
-Pygments>=2.0.2
-requests>=2.6.0
-docutils>=0.12
-pyxdg>=0.25
-pymongo>=3.0
-Markdown>=2.6.1
-python-dateutil>=2.4.2
-pytz>=2015.6
+Flask==0.12.2
+Werkzeug==0.14.1
+PyYAML==3.12
+Pygments==2.2.0
+requests==2.18.4
+docutils==0.14
+pyxdg==0.26
+pymongo==3.6.0
+Markdown==2.6.11
+python-dateutil==2.6.1
+pytz==2018.3