Remove pygraphviz as requirement

Change-Id: I1ea03904d88304ad9e85031ea24f08664425b8dc
diff --git a/requirements.txt b/requirements.txt
index 4f05fbb..8f7d6a7 100755
--- a/requirements.txt
+++ b/requirements.txt
@@ -22,7 +22,6 @@
 pep8-naming>=0.3.3
 protobuf-to-dict>=0.1.0
 pyflakes>=1.0.0
-pygraphviz>=1.3.1
 pylint>=1.5.2
 #pypcap>=1.1.5
 pyOpenSSL>=0.13