Swap simple `gsub` for `tr`

TIL that `tr` is substantially faster than `gsub`.  Whilst this commit won't
shake the earth it'll add a small something to performance.

`gsub` vs `tr`:

```
   gsub   311878.2 (± 3.5%) i/s
   tr    1573891.1 (± 4.6%) i/s
```
Merge pull request #75 from neilmiddleton/tr_is_faster

Swap simple `gsub` for `tr`