Use "ghc-mod version" to detect version instead of get_last_errmsg

I was having trouble using ghcmod-vim on NixOS and tracked it down to the fact ghc-mod gets wrapped by a shell script for which vimproc#get_last_errmsg() returns nothing. This change calls 'ghc-mod version' to extract the version number instead which works with the wrapper.

Note: my VimL skills are weak so there may be a better way of doing this :)