Use `(( ))` to do numerical comparison

Here's why:

    [[ "100" < 20 ]] # false
    (( "100" < 20 )) # true

`[[ ]]` does a string comparison, which puts `20` as higher than `"100"` by
comparing the first letter. `2` and `3` are also "greater than" 100. `(( ))`
correctly compares them as numbers.

More on arithmetic expressions:
http://robots.thoughtbot.com/the-unix-shells-humble-if