Switch to webcrypto entropy

This pull request switches ZeroBin to use the WebCrypto API for seeding SJCL's entropy. If the webcrypto API is not present, the old "startCollectors" method is used.

This fixes the build.

Also, I figured out how to have multiple secured SauceLabs access tokens in the .travis.yml file simultaneously. Travis will only decrypt the access token if the code originates from the proper fork. So make sure you [add your Sauce access tokens](https://docs.saucelabs.com/ci-integrations/travis-ci/)!