Update requirements.txt (#58)

The requirement.txt has version mismatch with setup.py.

requirements.txt

@@ -48,7 +48,7 @@ pyparsing==2.2.0
 six==1.11.0
 sqlalchemy==1.3.0
 wheel==0.31.1
-neo4j-driver==1.6.0
+neo4j-driver==1.7.2
 neotime==1.0.0
 pytz==2018.4
 antlr4-python2-runtime==4.7.1