Change the mocha dependency to be a peer dependency.

This changes the dependency on `mocha` to a so-called `peerDependency`.

Most people want to have `grunt-mocha-test` installed alongside `mocha`, so they
can use the `mocha` command line utility and the grunt test runner. Specifying `mocha`
as a `peerDependency` makes sure only one mocha version is installed and used.