Support new JDBC configuration style.

Please see: https://github.com/jruby/activerecord-jdbc-adapter

activerecord-jdbc-adapter supports a newer style database.yml configuration that accepts just, "jdbc" as the adapter type rather than "jdbcmysql". It then requires a database-specific jdbc connection string. There are certain cases where one must use this new syntax. See below for an example of the database.yml syntax.

This pull request adds support for this new style jdbc config, and includes passing specs. Thank you!

```ruby
development:
  adapter: jdbc
  username: blog
  password:
  driver: com.mysql.jdbc.Driver
  url: jdbc:mysql://localhost:3306/weblog_development
```