Breaking: bump level-js from 5.x to 6.x (#194)

Removes legacy range options and drops support of IE11 and Safari 9-11.

Author: achingbrain

package.json

@@ -14,7 +14,7 @@
   },
   "browser": "browser.js",
   "dependencies": {
-    "level-js": "^5.0.0",
+    "level-js": "^6.0.0",
     "level-packager": "^5.1.0",
     "leveldown": "^5.4.0"
   },