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
```
Allow groups to contain the master shard
Allow default for migration using_group() function

One approach to sharding is to have many databases with
identical schemas, which are not replicas of each other.
In this case always want to run all migrations on all of
the shards without needing to explicitly state this in
your migration files with the `using_group` syntax.

This patch allows you to specify a default_migration_group
setting in your shards.yml, so that all migrations will run
against all shards in the specified group, unless a different
`using_group` call is explicitly made.
Make shard-friendly clear_query_cache implementation less convoluted
Shard-friendly implementations of clear_{active/all}_connections!
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