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
Merge pull request #213 from nickmarden/nick/default_migration_group

Allow default for migration using_group() function
Merge pull request #214 from nickmarden/nick/clear_connections

Make it possible to clear connections correctly
Replace pry with pry-debugger
Fix a spec that worked, but should not have
Show that multiple calls to selector methods, e.g. .all, cause wrong-shard behavior
Get rid of "double select" bug by wrapping all association queries in using() blocks