Added PHP 7.3 support - updated Travis CI configuration

[michalbundyra]

No description provided.

[tux-rampage]

This is causing Build failures. There seems to be a Conding-Bug that was not yet spotted. I'll take a look at it later.

@weierophinney & @Ocramius does this need to be fixed first before this can be merged or merge and new PR with fix?

[michalbundyra]

@tux-rampage The problem is in zend-stdlib 2.7.7. I don't know what we are going to do with it. Technically PHP 7.3 is supported with latest dependencies. zend-stdlib v2 doesn't seem to be supported... Not sure how we can resolve failing build here - maybe remove "lowest" build from docker on PHP 7.3?

[weierophinney]

Considering the component now requires PHP 7.1, there's no reason to support zend-stdlib 2.7 any longer; just drop that version from the constraint, and you should be good to go.

[michalbundyra]

@tux-rampage PR is now updated