.vimrc: Use Selenized instead of Solarized

Selenized is a similar colorscheme to Solarized, but mapping to ANSI
color codes with less side effects. Note that the Selenized palatte
only has sixteen colors so we should tell vim to not attempt to make
a 256-color approximation.

https://github.com/jan-warchol/selenized/issues/65

.vimrc

@@ -12,7 +12,9 @@ filetype plugin on      " enable file type plugins
 filetype indent on      " enable file type indents
 
 set background=dark
-colorscheme solarized
+" restrict vim to 16 ANSI colors in the palette instead of approximating 256
+set t_Co=16
+colorscheme selenized
 
 " for per-directory vimrc files
 " see: https://jezenthomas.com/how-i-write-invoices-in-vim/