chore(package): update rollup-plugin-terser to version 6.0.0 (#313)

Co-authored-by: greenkeeper[bot] <23040076+greenkeeper[bot]@users.noreply.github.com>

package.json

@@ -65,7 +65,7 @@
     "rollup-plugin-babel": "^4.0.3",
     "rollup-plugin-commonjs": "^10.0.0",
     "rollup-plugin-node-resolve": "^5.0.0",
-    "rollup-plugin-terser": "^5.0.0",
+    "rollup-plugin-terser": "^6.0.0",
     "sinon": "^9.0.0",
     "typescript": "^3.5.1"
   }