chore(package): update eslint to version 5.15.3

Closes #34

Author: greenkeeper[bot]

package.json

@@ -21,7 +21,7 @@
     "babel-preset-es2017": "^6.24.1",
     "babel-preset-flow": "^6.23.0",
     "codecov": "^3.0.0",
-    "eslint": "^4.3.0",
+    "eslint": "^5.15.3",
     "eslint-plugin-flowtype": "^2.34.0",
     "flow-bin": "^0.75.0",
     "greenkeeper-lockfile": "1",