Travis: test coverage on the highest supported PHP version

As PHP 7.3 has been released, the build to test code coverage should be run on PHP 7.3.

In contrast to when the PHP 7.3 build was added, there does appear to be a code coverage driver available for PHP 7.3 by now, so we should be save to make this change.

Ref: 764

.travis.yml

@@ -20,7 +20,7 @@ php:
   - 5.6
   - 7.0
   - 7.1
-  - 7.3
+  - 7.2
   - nightly
 
 env:
@@ -38,13 +38,13 @@ matrix:
   include:
     # N.B.: Coverage is only checked on the lowest and highest stable PHP versions for all PHPCS versions.
 
-    - php: 7.2
+    - php: 7.3
       env: PHPCS_VERSION="dev-master" LINT=1 SNIFF=1 COVERALLS_VERSION="^2.0"
       addons:
         apt:
           packages:
             - libxml2-utils
-    - php: 7.2
+    - php: 7.3
       env: PHPCS_VERSION="2.3.0" COVERALLS_VERSION="^2.0"
 
     # PHPCS 3.x cannot be run on PHP 5.3.