Highlight elixir module declarations

The following commit in vim-elixir caused modules in module declarations
to no longer be highlighted yellow.

https://github.com/elixir-lang/vim-elixir/commit/babcd09769a2d45fbd93aed326c959d1e1a35523

This adds a `elixirModuleDeclaration` highlight explicitly to make the
modules yellow again.

colors/gruvbox.vim

@@ -1122,6 +1122,8 @@ hi! link elixirDocString Comment
 hi! link elixirStringDelimiter GruvboxGreen
 hi! link elixirInterpolationDelimiter GruvboxAqua
 
+hi! link elixirModuleDeclaration GruvboxYellow
+
 " }}}
 " Scala: {{{