Apply #88 changes to src/cell.js

#88 mistakenly applied the changes to `lib/backgrid.js` but not to `src/cell.js`. This PR fixes that error by applying the change to `src/cell.js`, and updating `lib/backgrid.min.js`.

Sorry about that! Can you confirm how things should be done -- should changes like the one made in #88 be made in `src/` and also always update the distribution files in `lib/`?