fix #22: 'hi Normal ctermbg=234' sets background=light

workaround for bug:
https://groups.google.com/forum/#!msg/vim_dev/afPqwAFNdrU/nqh6tOM87QUJ
Merge pull request #26 from justinmk/master

fix #22: 'hi Normal ctermbg=234' sets background=light