Change let into const (#457)

This patch:
- changes `let` into `const` throughout codebase
- adds eslint check to prefer const over let

lib/Launcher.js

@@ -113,7 +113,7 @@ class Launcher {
    * @return {!Promise<!Browser>}
    */
   static async connect({browserWSEndpoint, ignoreHTTPSErrors = false}) {
-    let connection = await Connection.create(browserWSEndpoint);
+    const connection = await Connection.create(browserWSEndpoint);
     return new Browser(connection, !!ignoreHTTPSErrors);
   }
 }
@@ -127,12 +127,12 @@ function waitForWSEndpoint(chromeProcess, timeout) {
   return new Promise((resolve, reject) => {
     const rl = readline.createInterface({ input: chromeProcess.stderr });
     let stderr = '';
-    let listeners = [
+    const listeners = [
       helper.addEventListener(rl, 'line', onLine),
       helper.addEventListener(rl, 'close', onClose),
       helper.addEventListener(chromeProcess, 'exit', onClose)
     ];
-    let timeoutId = timeout ? setTimeout(onTimeout, timeout) : 0;
+    const timeoutId = timeout ? setTimeout(onTimeout, timeout) : 0;
 
     function onClose() {
       cleanup();