Update requirements to include pytz. (see below)

This is an *extremely* common module. I think 
including it shouldn’t be a problem.

Author: theengineear

requirements.txt

@@ -7,3 +7,9 @@
 
 ## http request dependencies ##
 requests==2.3.0
+
+## python 2 to 3 compatibility ##
+six==1.8.0
+
+## timezone definitions ##
+pytz==2014.9

setup.py

@@ -40,5 +40,5 @@ def readme():
                 'plotly/matplotlylib/mplexporter',
                 'plotly/matplotlylib/mplexporter/renderers'],
       package_data={'plotly': ['graph_reference/*.json']},
-      install_requires=['requests', 'six'],
+      install_requires=['requests', 'six', 'pytz'],
       zip_safe=False)