add tornado to requirements

Author: timkpaine

requirements.txt

@@ -1,20 +1,21 @@
-Bottleneck==1.2.0
-coverage==4.3.4
-cycler==0.10.0
-gdax==1.0.6
-krakenex==0.1.3
-matplotlib==2.0.2
-mock==2.0.0
-nose==1.3.7
-numexpr==2.6.2
-numpy==1.13.0
-pandas==0.19.2
-pbr==1.10.0
-pyparsing==2.1.10
-python-dateutil==2.6.0
-pytz==2016.10
-requests==2.13.0
-scipy==0.18.1
-seaborn==0.7.1
-six==1.10.0
-websocket-client==0.40.0
+Bottleneck>=1.2.0
+coverage>=4.3.4
+cycler>=0.10.0
+gdax>=1.0.6
+krakenex>=0.1.3
+matplotlib>=2.0.2
+mock>=2.0.0
+nose>=1.3.7
+numexpr>=2.6.2
+numpy>=1.13.0
+pandas>=0.19.2
+pbr>=1.10.0
+pyparsing>=2.1.10
+python-dateutil>=2.6.0
+pytz>=2016.10
+requests>=2.13.0
+scipy>=0.18.1
+seaborn>=0.7.1
+six>=1.10.0
+tornado>=4.5.2
+websocket-client>=0.40.0