Remove grunt webdriver task for pull requests

[eKoopmans]

Bug: Grunt webdriver task for pull requests

The webdriver task in the Grunt testing process doesn't work for external pull requests, and is causing all PRs to fail.

Fix

The webdriver task is set up to connect to Sauce Labs, which hosts the Selenium cloud for browser testing. To connect to Sauce Labs, tests/utils.js looks for the environment variables SAUCE_USERNAME and SAUCE_ACCESS_KEY.

Unfortunately, those variables are encrypted in .travis.yml (as they should be), which means that they are not available to pull requests from forks. Thus the test fails, because it never connects to Sauce Labs.

To prevent this guaranteed failure, I have disabled the webdriver task if it detects that it is being performed in a pull request, as recommended in the Travis documentation:

To work around this, restrict these tests only to situations where the environment variables are available, or disable them for pull requests entirely.

Related issues/pull requests

#782, #941, #1084, #1085, #1086, #1087, #1088

(And really every open pull request, since none can currently pass with the current setup).

[niklasvh]

Fixed with https://github.com/niklasvh/html2canvas/blob/v1.0.0/karma.conf.js#L6