[polymer-cli] Selenium failed for running test locally and on CI

[motss]

When running test locally for both Chrome and Firefox with polymer-cli, it fails to download the required binary for Firefox.

# error log
Installing and starting Selenium server for local browsers
----------
selenium-standalone installation starting
----------

---
selenium install:
from: https://selenium-release.storage.googleapis.com/3.12/selenium-server-standalone-3.12.0.jar
to: /home/circleci/app-datepicker/node_modules/polymer-cli/node_modules/selenium-standalone/.selenium/selenium-server/3.12.0-server.jar
---
chrome install:
from: https://chromedriver.storage.googleapis.com/2.43/chromedriver_linux64.zip
to: /home/circleci/app-datepicker/node_modules/polymer-cli/node_modules/selenium-standalone/.selenium/chromedriver/2.43-x64-chromedriver
---
firefox install:
from: https://github.com/mozilla/geckodriver/releases/download/v0.23.0/geckodriver-v0.23.0-linux64.tar.gz
to: /home/circleci/app-datepicker/node_modules/polymer-cli/node_modules/selenium-standalone/.selenium/geckodriver/0.23.0-x64-geckodriver

Error: Could not download https://github.com/mozilla/geckodriver/releases/download/v0.23.0/geckodriver-v0.23.0-linux64.tar.gz