travis: add rpc backend test to test targets

Makefile

@@ -14,6 +14,7 @@ create-sandbox:
 test: haskell-backend-test
 test: ivory-backend-test
 test: tower-backend-test
+test: rpc-backend-test
 
 haskell-backend-test:
 	cabal run gidl -- -b haskell \