Add Cap v3 note to README. Pin versions in Gemspec.

ref: #3 

I added some lines to the `README.md` and pinned the versions in the gemspec.

This way a `gem install` or a `bundle install` on an unsupported version will prevent users from getting errors that incompatibilities cause, it will instead show:

```bash
Bundler could not find compatible versions for gem "capistrano":
  In Gemfile:
    capistrano-new-relic (>= 0) ruby depends on
      capistrano (< 3.0, >= 2.0) ruby

    capistrano (3.1.0)
```

You'll need to version bump and push a new gem release for this to take effect though.