Improve README: unnecessary `require` in Gemfile

Because there is [lib/ar-octopus.rb](https://github.com/tchandy/octopus/blob/master/lib/ar-octopus.rb) and it requires octopus,we don't have to write `require: "octopus"` by the hand. I just checked it by the following steps:

```
$ rails new example --skip-bundle
$ cd example
$ echo 'gem "ar-octopus", "0.8.0"' >> Gemfile
$ bundle
$ rails runner "puts Octopus.present?"
true
```