Make some smart-mode-line info visible in light theme (#137)

The current color for the sml/global face (used for rendering e.g.
column indicator, minor mode names) was gruvbox-burlywood4. This color
doesn't invert and it's invisible in a focused window when using the
light hard variant.

This fixes this by using gruvbox-light4, which will invert correctly
and display properly in light themes.

gruvbox.el

@@ -374,7 +374,7 @@ Should contain 2 %s constructs to allow for theme name and directory/prefix")
      (elfeed-search-filter-face                 (:inherit 'font-lock-string-face))
 
      ;; smart-mode-line
-     (sml/global                                (:foreground gruvbox-burlywood4 :inverse-video nil))
+     (sml/global                                (:foreground gruvbox-light4 :inverse-video nil))
      (sml/modes                                 (:foreground gruvbox-bright_green))
      (sml/filename                              (:foreground gruvbox-bright_red :weight 'bold))
      (sml/prefix                                (:foreground gruvbox-light1))