Assorted fixes for small bugs in master

[diosmosis]

I was trying to see if I could get webdrivercss to work w/ the latest webdriverio. Then I realized a rewrite of webdrivercss is currently in progress, and the work was probably more than I could do in a couple of hours.

I managed to find and fix a couple bugs, though, while trying to get the instantiation.js test to pass. I figured they might be useful, so I created this PR.

Changes included:

• Don't use the es2015 preset since it requires the generator transform. Since Babel 6, the blacklist property is no longer supported in .babelrc, so I think the only alternative is to specify manually what transforms to use.
• In the gruntfile, the loadTasks('build') call would cause grunt to try and load tasks from the build directory. If the compiled files are in build, this causes grunt to fail.
• Made documentScreenshot.js export it's function as the default export (this is what webdriver.css was expecting).
• Don't require ErrorHandler directly from lib/.../ErrorHandler since this will try and load webdriverio's ES6 JS instead of the compiled JS.
• Make sure there is an applitools key in DEFAULT_PROPERTIES since many files expect it to be there (eg, workflow.js).
• In afterHook in bootstrap.js, don't pass a callback to browser.end() since it returns a promise now.
• Get parts of the instantiation spec to pass. I couldn't get all of them to pass since it calls WebDriverCSS.init() multiple times (resulting in 'command already defined' errors). I believe the best way to fix this is to isolate the tests in that spec more, but I didn't want to make this PR any bigger.

If the PR requires some changes, let me know.

[diosmosis]

@christian-bromann don't want to create any noise or be annoying, but I'd be interested in helping to get webdrivercss compatible w/ webdriver.io 4. Would it be possible for me to get involved in that effort?