fix(Puppeteer): checkRequirements failed when use only puppeteer-firefox

[Vorobeyko]

When require Puppeteer with puppeteer-firefox and not install puppeteer - I obtains exception:

Could not load helper Puppeteer from module '...':
Cannot read property 'browser' of undefined

[DavertMik]

I think checkRequirements won't work for both packages puppeteer and puppeteer-firefox.
A proper solution would be to create PuppeteerFirefox helper which extends from Puppeteer.
Then we can easily override _checkRequirements.