fix #163 - prefer current pair over previous pair

[ryfow]

I think this resolved #163 by only moving back when we're not in a valid pair.

I'm having trouble getting c-mode and org-mode features to run with or without this change though, so I might be missing some test failure.

[coveralls]

Coverage increased (+0.02%) to 85.091% when pulling f756393 on ryfow:current-pair into cfef9aa on magnars:master.

[Wilfred]

Yep, see #211. Tests are currently only passing on Emacs 24.3.

[coveralls]

Coverage increased (+0.02%) to 85.154% when pulling 019a1fe on ryfow:current-pair into 2357f1d on magnars:master.

[magnars]

Thanks!