Fix error when running `run-dev` after `clean` target

Makefile

@@ -105,6 +105,7 @@ run: ${BINARY}
 	./chronograf
 
 run-dev: chronogiraffe
+	mkdir -p ui/build
 	./chronograf -d --log-level=debug
 
 clean:

ui/package.json

@@ -11,7 +11,7 @@
   "scripts": {
     "start": "node parcel.js",
     "build": "parcel build -d build --no-source-maps --public-url '' src/index.html",
-    "clean": "rm -rf ./build && rm -rf ./.cache",
+    "clean": "rm -rf ./build/* && rm -rf ./.cache",
     "test": "jest",
     "test:watch": "jest --watch",
     "test:debug": "node --inspect-brk node_modules/.bin/jest --runInBand --watch",