Update string-width from ^2.1.1 to 3.0.0 (#76)

Author: dependencies[bot]

package.json

@@ -60,6 +60,6 @@
     "extract-comments": "^1.1.0",
     "prettier": "^1.15.2",
     "solidity-parser-antlr": "^0.3.1",
-    "string-width": "^2.1.1"
+    "string-width": "3.0.0"
   }
 }