skip example image processing in make_gallery.py when pygraphviz not available
skip example image processing in make_gallery.py when pygraphviz not available

--HG--
extra : convert_revision : svn%3A3ed01bd8-26fb-0310-9e4c-ca1a4053419f/networkx/trunk%401230

jni/networkx,wasade/networkx,goulu/networkx,jfinkels/networkx,farhaanbukhsh/networkx,ghdk/networkx,dhimmel/networkx,harlowja/networkx,michaelpacer/networkx,kai5263499/networkx,jtorrents/networkx,debsankha/networkx,beni55/networkx,sharifulgeo/networkx,nathania/networkx,yashu-seth/networkx,jtorrents/networkx,SanketDG/networkx,dhimmel/networkx,kernc/networkx,debsankha/networkx,chrisnatali/networkx,NvanAdrichem/networkx,aureooms/networkx,nathania/networkx,ionanrozenfeld/networkx,kai5263499/networkx,farhaanbukhsh/networkx,Sixshaman/networkx,RMKD/networkx,ghdk/networkx,blublud/networkx,ltiao/networkx,sharifulgeo/networkx,RMKD/networkx,cmtm/networkx,blublud/networkx,sharifulgeo/networkx,jni/networkx,OrkoHunter/networkx,debsankha/networkx,kai5263499/networkx,bzero/networkx,jni/networkx,RMKD/networkx,jakevdp/networkx,nathania/networkx,dhimmel/networkx,dmoliveira/networkx,ionanrozenfeld/networkx,bzero/networkx,JamesClough/networkx,ionanrozenfeld/networkx,farhaanbukhsh/networkx,ghdk/networkx,jakevdp/networkx,jakevdp/networkx,harlowja/networkx,harlowja/networkx,chrisnatali/networkx,aureooms/networkx,andnovar/networkx,kernc/networkx,kernc/networkx,tmilicic/networkx,aureooms/networkx,dmoliveira/networkx,bzero/networkx,jcurbelo/networkx,chrisnatali/networkx,blublud/networkx,dmoliveira/networkx