Remove SerializableResource.serialize in favor of .new

[bf4]

Per discussion in #1098

[bf4]

Failures on appveyour are race conditions we haven't fixed yet

• https://ci.appveyor.com/project/joaomdmoura/active-model-serializers/build/49/job/pf3g0vp1otqecv42
• https://ci.appveyor.com/project/joaomdmoura/active-model-serializers/build/49/job/j0tijlxbeluf9r5e

 1) Failure:
ActionController::Serialization::ImplicitSerializerTest#test_cache_expiration_on_update [C:/projects/active-model-serializers/test/action_controller/serialization_test.rb:394]:
--- expected
+++ actual
@@ -1 +1 @@
-"{\"id\":1,\"title\":\"ZOMG a New Post\",\"body\":\"Body\",\"comments\":[{\"id\":1,\"body\":\"ZOMG A COMMENT\"}],\"blog\":{\"id\":999,\"name\":\"Custom blog\"},\"author\":{\"id\":1,\"name\":\"Joao Moura.\"}}"
+"{\"id\":1,\"title\":\"New Post\",\"body\":\"Body\",\"comments\":[{\"id\":1,\"body\":\"ZOMG A COMMENT\"}],\"blog\":{\"id\":999,\"name\":\"Custom blog\"},\"author\":{\"id\":1,\"name\":\"Joao Moura.\"}}"

[NullVoxPopuli]

👍

[beauby]

Should we put a deprecation warning until 0.10 is released, rather than suppressing the method?

[joaomdmoura]

I don't think we needed the warning because it was not even on the RC2, but I'm not against it if you think we should.
But I think we should update/add some docs about it on our new docs for sure.

[beauby]

Oh, right. Let's just remove it then.

[NullVoxPopuli]

@bf4 are you finished with this?