changed to arrow fn and updated travis config.

.travis.yml

 language: node_js
 
 node_js:
-  - lts/*
+  - "lts/*"
 
 install:
   - npm install

index.js

   });
 }
 
-const getFileThatExist = async function(...files) {
+const getFileThatExist = async (...files) => {
   for (const file of files) {
     if (file && await fileExist(file) === true) {
       return file;
   return false;
 }
 
-const filesExist = function(...files) {
+const filesExist = (...files) => {
   return Promise.resolve(!!getFileThatExist(...files));
 }