Remove 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.

We can check if it works by the following steps:

```sh
rails new example --skip-bundle
cd example
echo 'gem "ar-octopus", "0.8.0"' >> Gemfile
bundle
rails runner "puts Octopus.present?" #=> true
```
Merge pull request #208 from r7kamura/readme

Improve README: unnecessary `require` in Gemfile