Revert to previous way of sourcing vim plugin

Refs #95.

powerline/bindings/vim/plugin/source_plugin.vim

@@ -5,10 +5,7 @@ if ! has('python') && ! has('python3')
 	finish
 endif
 
-python <<EOF
-from __future__ import absolute_import
-import sys, vim
-sys.path.append(vim.eval('expand("<sfile>:h:h:h:h:h")'))
-from powerline.bindings.vim import source_plugin
-source_plugin()
-EOF
+python import sys, vim
+python sys.path.append(vim.eval('expand("<sfile>:h:h:h:h:h")'))
+
+source <sfile>:h:h/powerline.vim