Skip building on focal if VERSION is 7.2 or 7.3

PHP 7.2 and 7.3 uses `freetype-config` to check whether or not
freetype is available. This no longer available on Focal, which
uses `pkg-config` for this purpose.

In addition, focal's `php` package is 7.4 already
https://packages.ubuntu.com/focal/php
so it seems low-risk to drop support for these older versions.

Author: BanzaiMan

.travis.yml

@@ -26,6 +26,7 @@ jobs:
               eval "$(phpenv init -)"
             fi
           fi
+      if: !(env(VERSION) =~ /^7\.[23]/)
     - dist: bionic
       language: minimal
       group: dev