Fix defunct keyboard selection

https://github.com/mathquill/mathquill/pull/261#issuecomment-37909258
https://github.com/mathquill/mathquill/commit/32a48e4a4f2cd3ecaedcf36faac5c84fe590acc9#commitcomment-5721473
I somehow forgot to update `seln.first === cursor.next` to
`seln.ends[L] === cursor[R]`. This caused the keyboard to only select in
one direction.