chore(package): update eslint to version 6.4.0

Closes #5

Author: greenkeeper[bot]

package.json

@@ -29,7 +29,7 @@
         "yargs": "^10.0.3"
     },
     "devDependencies": {
-        "eslint": "^4.3.0"
+        "eslint": "^6.4.0"
     },
     "peerDependencies": {
         "wiring-pi": "^2.2.1"