Avoid regexp error for foo(*args).baz pattern

[todesking]

to reproduce:

foo(*args). # invoke omni completion here

[AndrewRadev]

I'm afraid I'm not very familiar with the completion script, but from what I was able to understand, it seems like issue #120 manages to fix it in a slightly safer way. With this PR, I'm not entirely sure if GetRubyVarType could be called with something that's not quite \w+, like @foo or something, which is why I decided to merge in @sencer's solution.

In any case, thank you for your work, and sorry for the long wait.