Remove the assumption of schema in DATABASE_URL

If you set the DATABASE_URL environment variable to `mydatabase` by accident, you end up getting a series of errors that are hard to trace. For example: 

```
warning: already initialized constant ActiveRecord::Base::OrmAdapter
```

Turns out the cascade of errors is due to the error raised by `.tr` being called on `nil`.

This commit makes sure that `scheme` is set before calling `.tr` on it. My previous iteration used `@uri.scheme.try(:tr, '-', '_')` but using the `&&` logical operator is a fair bit faster: http://stackoverflow.com/questions/26655032/try-vs-performance

With this change, the error message becomes much more understandable:

```
FATAL:  database "mydatabase" does not exist (ActiveRecord::NoDatabaseError)
```

activerecord/lib/active_record/connection_adapters/connection_specification.rb

@@ -33,7 +33,7 @@ class ConnectionUrlResolver # :nodoc:
         def initialize(url)
           raise "Database URL cannot be empty" if url.blank?
           @uri     = uri_parser.parse(url)
-          @adapter = @uri.scheme.tr('-', '_')
+          @adapter = @uri.scheme && @uri.scheme.tr('-', '_')
           @adapter = "postgresql" if @adapter == "postgres"
 
           if @uri.opaque