removed cytoscape from anonymous function call

Hi, Max:
I noticed that if I use cytoscape via require('cytoscape'), the cytoscape-qtip doesn't work very well, because the anonymous call expects `cytoscape` to be defined as global variable, which, if you use requirejs, won't be defined globally. I had a look at cytoscape-context-menu you wrote and did some change as per that project to fix the issue. Please have a look.