Update TF_VERSION to fix CI Action workflow (keras-team#301)

Author: albertvillanova

Makefile

@@ -1,5 +1,5 @@
 container-test:
-	docker build --build-arg TF_VERSION=2.3.0-rc0 -t keras-io .
+	docker build --build-arg TF_VERSION=2.4.0-rc0 -t keras-io .
 	docker run --rm -d -p 8000:8000 --name keras-io-server keras-io
 	sleep 10
 	docker exec keras-io-server echo I am alive

requirements.txt

@@ -6,5 +6,5 @@ mdx_truly_sane_lists
 sphinx~=3.0.3
 black==19.10b0
 pathlib
-tf-nightly~=2.3.0.dev20200610
+tf-nightly~=2.4.0.dev20201023
 PyYAML