no need for graphviz library anymore

(also it was licensed as GPL)
diff --git a/Gemfile b/Gemfile
index 43029d0..066fc72 100644
--- a/Gemfile
+++ b/Gemfile
@@ -57,9 +57,6 @@
 gem 'workflow_parser', github: 'myExperiment/workflow_parser'
 gem 'taverna-t2flow', github: 'myExperiment/workflow_parser-t2flow'
 
-# A gem to build digraphs
-gem 'ruby-graphviz'
-
 # A gem to query the prov.ttl
 gem 'sparql'