Updated Travis CI configuration

[michalbundyra]

• removed Deploy Docs residues
• removed HHVM build
• moved cs-check and test coverage to run on PHP 7.1 with locked deps

[weierophinney]

Marking for 2.11.0, as we cannot remove HHVM support within a bugfix release.

[michalbundyra]

@weierophinney Changed target to develop then, will rebase it shortly.

[weierophinney]

Thanks, @webimpress.