Build/Test Tools: Add PHP7.3 RC1 to the build matrix

Currently, we need to do some extra steps to run 7.3, but this will help us with testing compatibility.

See: https://github.com/travis-ci/travis-ci/issues/9717

See #44771.

Built from https://develop.svn.wordpress.org/trunk@43673


git-svn-id: https://core.svn.wordpress.org/trunk@43502 1a063a9b-81f0-0310-95a4-ce76da25c4cd