[DRE-maint] ruby-graphviz is marked for autoremoval from testing

[Debian testing autoremoval watch]

ruby-graphviz 1.2.2-1 is marked for autoremoval from testing on 2018-02-21

It is affected by these RC bugs:
888150: ruby-graphviz: FTBFS on ruby2.5:  undefined method `graph' for nil