vimPlugins: change wombat256 source to github

See issue #8694. Previously wombat256 was downloading its source from
the vim website using fetchurl. The builder was failing when it tried to
unpack the .vim source file. Using the vim-scripts github repo avoids
this problem.

pkgs/misc/vim-plugins/vim-plugin-names

@@ -57,6 +57,7 @@
 "github:vim-scripts/a.vim"
 "github:vim-scripts/align"
 "github:vim-scripts/tabmerge"
+"github:vim-scripts/wombat256.vim"
 "github:wakatime/vim-wakatime"
 "github:wincent/command-t"
 "matchit.zip"
@@ -97,4 +98,3 @@
 "vim-snippets"
 "vim2hs"
 "vundle"
-"wombat256"