replace iunmap <CR> with inoremap <CR> <CR>

If noninvasive completion is set, i get the following error:

Error detected while processing ~/.vimrc:
line  730:
E31: No such mapping
Press ENTER or type command to continue

To avoid this "no such mapping" error, instead of unmapping the commands for <CR> one can simply overwrite it with itself to reset it, which works regardless of wether the mapping existed or not.
Merge pull request #669 from wolfgangpfnuer/patch-1

replace iunmap <CR> with inoremap <CR> <CR>