Add cymem to requirements_rtd.

requirements_rtd.txt

@@ -6,6 +6,7 @@ PyYAML>=3.13
 arrow>=0.10.0
 click-log>=0.2.1
 click>=6.7
+cymem>=2.0
 gevent>=1.3.6
 gunicorn>=19.7.1
 pytest-flask