Delete duplicate 'mutant' gem

This might be unnecessary, but I figured I'd start a conversation (because I'll certainly learn something).

When I run bundle install I get the following warning:

```plain
Your Gemfile lists the gem mutant (>= 0) more than once.
You should probably keep only one of them.
While it's not a problem now, it could cause errors if you change the version of just one of them later.
```

My guess is that it's because the Gemfile references the gemspec, and that perhaps the gemspec implicitly requires the gem defined within it.