Update strip-ansi from ^3.0.0 to ^4.0.0 (#10)

Includes a breaking change chalk/strip-ansi@0978944 but doesn't affect strip-ansi, which has already dropped support for Node.js < 4.

Author: shinnn

package.json

@@ -46,7 +46,7 @@
   ],
   "dependencies": {
     "is-fullwidth-code-point": "^2.0.0",
-    "strip-ansi": "^3.0.0"
+    "strip-ansi": "^4.0.0"
   },
   "devDependencies": {
     "ava": "*",