[#5815290] Fix decimals

*Depends on https://github.com/thinkthroughmath/ttm-coffeescript-math/pull/4, but will work without merging.*

Allows only one decimal per numeric expression.

#### To Test

- Clone repo
- Install dependencies
- Make sure ttm-coffeescript-math is updated by running `bower update`
- [ ] run grunt test, ensure all passing (we don't have travis set up for these repos)
- [ ] Play with the calculator to check that decimal behavior is correct, especially: 
    - [ ] Only adds one decimal to a number
    - [ ] creates `0.` when pressed when no number is present