Take control of download URL

There have been multiple reports of failure with the existing URL,
though I can't reproduce.

Closes tpope#128.

README.markdown

@@ -9,8 +9,7 @@ private directories.
 Install to `~/.vim/autoload/pathogen.vim`.  Or copy and paste:
 
     mkdir -p ~/.vim/autoload ~/.vim/bundle; \
-    curl -LSso ~/.vim/autoload/pathogen.vim \
-        https://raw.github.com/tpope/vim-pathogen/master/autoload/pathogen.vim
+    curl -LSso ~/.vim/autoload/pathogen.vim https://tpo.pe/pathogen.vim
 
 If you're using Windows, change all occurrences of `~/.vim` to `~\vimfiles`.