Update Travis CI build matrix for PHP 7.2

[netweb]

Via ​https://core.trac.wordpress.org/ticket/40109#comment:15

FYI: PHP 7.2 alpha has been tagged and is now ​available on Travis as 7.2.

• PHP 7.2 compatibility is only being added to WordPress 4.9 at this stage (there is no current plans to backport for WordPress 4.8 or earlier) so this job should only test against WP master

• As PHP 7.2 has not yet been released and there are current PHP 7.1 test failures adding this 7.2 job should be allowed to fail

• PHP nightly on Travis CI has now become PHP 7.3.0-dev and no change is required to include PHP 7.3 in allowed failures, see ​https://travis-ci.org/buddypress/BuddyPress/jobs/257217162#L447

[netweb]

In 11649:

Build Tools: Run Travis CI tests on PHP 7.2, and WordPress 4.9 (master) allowing to fail.

Fixes #7574. See #7392.

[netweb]

In 11652:

Build Tools: Check for existence xdebug.ini using phpenv for Travis CI

This changeset is a follow up to [11649] where XDebug wasn't being detected correctly for the PHP 7.2 and 7.3 jobs

See #7574.