Update package.json

package.json

@@ -54,7 +54,7 @@
     "mocha": "^6.2.0",
     "mocha-chrome": "^2.0.0",
     "node-sass": "^4.12.0",
-    "rollup": "1.21.2",
+    "rollup": "^1.21.2",
     "rollup-plugin-babel": "^4.3.3",
     "rollup-plugin-commonjs": "^10.1.0",
     "rollup-plugin-eslint": "^7.0.0",
@@ -67,4 +67,4 @@
     "servor": "^2.2.1",
     "travis-size-report": "^1.1.0"
   }
-}
+}