Made SyntacticIsVersionAtLeast more robust

It now handles more cases, which will cause Syntastic to degrade better
when commands return weird versions (e.g. the command crashes).
Renamed version check function

Moved the `SyntasticIsVersionAtLeast()` into `util.vim` as
`syntastic#util#versionIsAtLeast()` as per @scrooloose's suggestion.
Merge pull request #546 from docwhat/ver-is-at-least

Made SyntacticIsVersionAtLeast more robust