Fix detection of version of ghc-mod.

Fix the following error:

Error detected while processing function ghcmod#util#check_version:
line    3:
E684: list index out of range: 0
E15: Invalid expression: a:version[l:i] > l:ghc_mod_version[l:i]
E684: list index out of range: 1
E15: Invalid expression: a:version[l:i] > l:ghc_mod_version[l:i]
E684: list index out of range: 2
E15: Invalid expression: a:version[l:i] > l:ghc_mod_version[l:i]