Remove gulp-a11y in favor of axe-webdriverjs

[ryuran]

Useless level of dependencies.
Use a11y https://github.com/addyosmani/a11y
or pa11y https://github.com/pa11y/pa11y

[okeul]

It's better to use pa11y :-)

[ryuran]

https://github.com/dequelabs/axe-webdriverjs