added php 7.2 and php nightly in travis

[kokspflanze]

• added php 7.2 and php nightly in travis
• removed hhvm in travis

[michalbundyra]

It should be CS_CHECK here, below (in line 82) we are checking:

if [[ $CS_CHECK == 'true' ]];

[weierophinney]

I plan to remove php:nightly from the build until the PHP project provides a 7.3 alpha; otherwise, we spend too much maintenance time chasing stuff that may change radically before the release.

[weierophinney]

Thanks, @kokspflanze