Fix for Emacs 24.1, 24.2, 24.3

'electric-pair-backward-delete-char-untabify' is introduced at
Emacs 24.4.
Add regression test for older emacs(<= 24.3)
Merge pull request #226 from defunkt/fix-for-electric-pair-mode

Fix for Emacs 24.1, 24.2, 24.3