Rebuild horse between each CI run so that it works for OTP/25

Makefile

@@ -60,6 +60,11 @@ include erlang.mk
 
 TEST_ERLC_OPTS += +'{parse_transform, eunit_autoexport}' +'{parse_transform, horse_autoexport}'
 
+# Always rebuild horse because OTP-25.0+ can't use the older build.
+
+ci-setup::
+	-$(MAKE) -C $(DEPS_DIR)/horse clean
+
 # Mimetypes module generator.
 
 GEN_URL = http://svn.apache.org/repos/asf/httpd/httpd/trunk/docs/conf/mime.types