fix: pin rollup to 1.9.2

Author: greenkeeper[bot]

package.json

@@ -97,7 +97,7 @@
     "coveralls": "^3.0.2",
     "istanbul": "^0.4.5",
     "npm-dollar": "^2.1.3",
-    "rollup": "^1.1.2",
+    "rollup": "1.9.2",
     "rollup-plugin-babel": "^4.3.2",
     "rollup-plugin-node-resolve": "^4.0.0",
     "tressa": "^0.3.1",