Build/Test Tools: Use 7.3 for PHP 7.3

Travis now supports PHP7.3 without workarounds, so let's remove the workarounds.

Merges [43726] to trunk.

See #44771.

Built from https://develop.svn.wordpress.org/trunk@43975


git-svn-id: http://core.svn.wordpress.org/trunk@43807 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.1-alpha-43974';
+$wp_version = '5.1-alpha-43975';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.