Rename Adapter.get to Adapter.lookup

Per https://github.com/rails-api/active_model_serializers/pull/1017#discussion_r39003855
comment by sandstrom in discussion of the inherited hook

> I'm thinking that it would be better to register adapters manually, without using the hook, i.e.
> have people call ActiveModel::Serializer::Adapter.register directly (perhaps in an initializer).

> Possibly, some inspiration can be taken from how ActiveJob adapters are wired[1].

> [1] a11571cec3/activejob/lib/active_job/queue_adapter.rb (L52-L56)

lib/active_model/serializer/adapter.rb

@@ -17,9 +17,9 @@ module ActiveModel
         klass.new(resource, options)
       end
 
-      # @see ActiveModel::Serializer::Adapter.get
+      # @see ActiveModel::Serializer::Adapter.lookup
       def self.adapter_class(adapter)
-        ActiveModel::Serializer::Adapter.get(adapter)
+        ActiveModel::Serializer::Adapter.lookup(adapter)
       end
 
       # Only the Adapter class has these methods.
@@ -49,7 +49,7 @@ module ActiveModel
         # @param  adapter [String, Symbol, Class] name to fetch adapter by
         # @return [ActiveModel::Serializer::Adapter] subclass of Adapter
         # @raise  [UnknownAdapterError]
-        def get(adapter)
+        def lookup(adapter)
           # 1. return if is a class
           return adapter if adapter.is_a?(Class)
           adapter_name = adapter.to_s.underscore