Exit gracefully in case python is not available.

This small patch lets the script exit quietly when python support is not
available rather then throwing multiple errors. This makes it easier to
share the vim configuration between different vim installations (eg.
macvim and the builtin /usr/bin/vim without python support on Mac OSX).

ftplugin/python/pyflakes.vim

@@ -10,6 +10,10 @@
 "
 " Maintainer: Kevin Watters <kevin.watters@gmail.com>
 " Version: 0.1
+if !has('python')
+    " exit if python is not available.
+    finish
+endif
 
 if exists("b:did_pyflakes_plugin")
     finish " only load once