fix #140 can't tab between math fields #herpderp #ImAnIdiot

apparently I was stupid in 664da8ae6a66dbb91ea0dd85916d3f52d3a421f7, I
did [this if block][1] but not this

[1]: https://github.com/mathquill/mathquill/commit/664da8ae6a66dbb91ea0dd85916d3f52d3a421f7#L0R90