Use processing-java to run sketches from Vim.

From Processing 2.0 beta 5, the 'use external editor' option has been removed.
Instead, a `processing-java` command is supplied to run sketches from the command
line. 

This commit updates vim-processing to use `processing-java` instead of the
previous AppleScript tool. `processing-java` can be installed by choosing 'Install
processing-java' from the Processing tools menu.

More info here:
https://forum.processing.org/topic/use-external-editor-is-gone-in-beta-5-now-what