Update gemma dependency to 3.0.0

This fixes a conflict with versions of bundler > 1.1.x.

Hopefully it doesn't break releasing the gem, but it seems to build OK.

Without this, running `bundle install` on devmac-1 gave this error:

```
Bundler could not find compatible versions for gem "bundler":
  In Gemfile:
    gemma (~> 2.2.0) ruby depends on
      bundler (~> 1.1.3) ruby

  Current Bundler version:
    bundler (1.2.2)

This Gemfile requires a different version of Bundler.
Perhaps you need to update Bundler by running `gem install bundler`?
```