[vim] fixed regression where bold was not active in gui

A test for t_Co < 16 also snagged the gui despite t_Co not
being defined in gui mode. Changed to t_Co == 8 for now,
which is the only known case requiring bold off. There may
be other t_Co values under 16 in low color terminals but I
haven't found them yet, so this should be sufficient.

colors/solarized.vim

@@ -410,7 +410,7 @@ endif
 "}}}
 " Overrides dependent on user specified values and environment "{{{
 " ---------------------------------------------------------------------
-if (g:solarized_bold == 0 || &t_Co < 16)
+if (g:solarized_bold == 0 || &t_Co == 8 )
     let s:b           = ""
     let s:bb          = ",bold"
 else