Fix color_name to g:color_name instead
This causes vim to properly reload the correct theme when needed.
Fixes issue #22
    
        | author | Sami Haahtinen <sami.haahtinen@hakosalo.fi> | 
    
        | date | Sun, 15 Nov 2015 09:51:00 +0200 | 
    
    
        | parents | db3707cbd870 | 
    
        | children | bd56b86c5403 | 
    
        | branches/tags | (none) | 
    
        | files | colors/badwolf.vim | 
Changes
    
--- a/colors/badwolf.vim	Fri Jul 19 20:55:06 2013 -0400
+++ b/colors/badwolf.vim	Sun Nov 15 09:51:00 2015 +0200
@@ -41,7 +41,7 @@
     syntax reset
 endif
 
-let colors_name = "badwolf"
+let g:colors_name = "badwolf"
 
 if !exists("g:badwolf_html_link_underline") " {{{
     let g:badwolf_html_link_underline = 1