Updates version string parsing, since version 5 does not output version info in error message.

As of ghc-mod version 5, running simply `ghc-mod` without argument gives:

`ghc-mod: No command given (try --help)`

rather than outputting the help info with the version info at the top. Instead of relying on that, ghc-mod vim should call `ghc-mod version` and parse the output of that. This also works with ghc-mod version 4.1.0 and above.

Syntastic solves this more holistically in order to support even older versions of ghc-mod, something else to consider:

https://github.com/scrooloose/syntastic/commit/d665fbf5641