fix: get-message isn't async

Remove the async designation from `get-message`, this function is purely
sync.

lib/get-message.js

@@ -1,6 +1,6 @@
 const _ = require('lodash')
 
-module.exports = async function (commitMessages, key, values) {
+module.exports = function (commitMessages, key, values) {
   if (!Object.prototype.hasOwnProperty.call(commitMessages, key)) {
     throw new Error(`Unknown message key '${key}'`)
   }