Test Heroku-16 stack (#406)

- Drop testing support for Cedar-10
- Add testing support for Heroku-16

makefile

-test: test-cedar-14
+test: test-heroku-16 test-cedar-14
+
+test-heroku-16:
+	@echo "Running tests in docker (heroku-16)..."
+	@docker run -v $(shell pwd):/buildpack:ro --rm -it -e "STACK=heroku-16" heroku/heroku:16 bash -c 'cp -r /buildpack /buildpack_test; cd /buildpack_test/; test/run;'
+	@echo ""
 
 test-cedar-14:
 	@echo "Running tests in docker (cedar-14)..."
 	@docker run -v $(shell pwd):/buildpack:ro --rm -it -e "STACK=cedar-14" heroku/cedar:14 bash -c 'cp -r /buildpack /buildpack_test; cd /buildpack_test/; test/run;'
 	@echo ""
 
-test-cedar-10:
-	@echo "Running tests in docker (cedar)..."
-	@docker run -v $(shell pwd):/buildpack:ro --rm -it -e "STACK=cedar" fabiokung/cedar bash -c 'cp -r /buildpack /buildpack_test; cd /buildpack_test/; test/run;'
-
 shell:
-	@echo "Opening cedar-14 shell..."
-	@docker run -v $(shell pwd):/buildpack:ro --rm -it heroku/cedar:14 bash -c 'cp -r /buildpack /buildpack_test; cd /buildpack_test/; bash'
+	@echo "Opening heroku-16 shell..."
+	@docker run -v $(shell pwd):/buildpack:ro --rm -it heroku/heroku:16 bash -c 'cp -r /buildpack /buildpack_test; cd /buildpack_test/; bash'
 	@echo ""