Dependencies upgrade

https://pypi.python.org/pypi/pyOpenSSL : fix memory leak
http://lxml.de/4.1/changes-4.1.1.html : speed improvement

requirements.txt

@@ -1,10 +1,10 @@
-certifi==2017.7.27.1
+certifi==2017.11.5
 flask==0.12.2
 flask-babel==0.11.2
-lxml==3.8.0
+lxml==4.1.1
 idna==2.5
 pygments==2.1.3
-pyopenssl==17.2.0
+pyopenssl==17.4.0
 python-dateutil==2.6.1
 pyyaml==3.12
 requests[socks]==2.18.4