Fixed calcul of REM font sizing

The operation `$ value / $ base + rem` returns a concatenated string and not a REM unit:

```shell
$ sass -i
>> 16 / 10 + rem
"16/10rem"
>> 16 / 10 + 0rem
1.6rem
```

An alternative is to use the https://github.com/ry5n/rem mixins that is more complete.