Fix not working UltiSnips completer

Fixes #820. Latest UltiSnips upstream changes was pretty huge. The most devastating change was that now UltiSnips uses a class instance of ``SnippetManager`` class for ``UltiSnips_Manager`` object instead of just imported module. 

This alone is pretty harmless since UltiSnips defines ``UltiSnips_Manager`` in global scope, but for some reason ( unknown for me ) its not defined in ``ycm_state._AddUltiSnipsDataIfNeeded`` scope ( even with ``global``. It should work with ``global``, I guess this is some kind of limitation of Vim python bindings... ). Because of that, I made changes to pass ``UltiSnips_Manager`` instance directly to ``OnBufferVisit`` call. Sorry, dont know better solution here :(

Also, I guess we should ping @SirVer, maybe he has a better solution.