travis: Update build script to test more reasonable configurations

PHP nightly recently got updated to dev-8.x, which is not supported
by any of the test scripts. This makes the test pretty useless and
doesn't help in any way.

Instead, the build script should focus on current versions of PHP,
starting from 5.6 to 7.3 (current stable release).

PHP 7.3 is a reasonable version to use for finding breaking changes
in the test scripts (phpunit especially warns about changes). These
tests can fail, of course.

.travis.yml

@@ -4,41 +4,43 @@ language: php
 install:
   - composer global require dealerdirect/phpcodesniffer-composer-installer;
   - composer global require phpcompatibility/php-compatibility;
-  # Use PHPUnit 6 for unit tests (stable), requires PHP 7
-  - if [[ $TRAVIS_PHP_VERSION == "7.0" ]]; then
-      composer global require phpunit/phpunit ^6;
-    fi
-  # Use latest PHPUnit on nightly to detect breaking changes
-  - if [[ $TRAVIS_PHP_VERSION == "nightly" ]]; then
-      composer global require phpunit/phpunit;
+  - if [[ "$PHPUNIT" ]]; then
+      composer global require phpunit/phpunit ^$PHPUNIT;
     fi
 
 script:
   - phpenv rehash
   # Run PHP_CodeSniffer on all versions
   - ~/.config/composer/vendor/bin/phpcs . --standard=phpcs.xml --warning-severity=0 --extensions=php -p;
-  # Check PHP compatibility for the lowest supported version
-  - if [[ $TRAVIS_PHP_VERSION == "5.6" ]]; then
+  # Check PHP compatibility for the lowest and highest supported version
+  - if [[ $TRAVIS_PHP_VERSION == "5.6" || $TRAVIS_PHP_VERSION == "7.3" ]]; then
       ~/.config/composer/vendor/bin/phpcs . --standard=phpcompatibility.xml --extensions=php -p;
     fi
-  # Run unit tests (stable)
-  - if [[ $TRAVIS_PHP_VERSION == "7.0" ]]; then
-      phpunit --configuration=phpunit.xml --include-path=lib/;
-    fi
-  # Run unit tests (latest/nightly)
-  # Check PHP compatibility for all versions, starting at the lowest supported version in order to detect breaking changes
-  - if [[ $TRAVIS_PHP_VERSION == "nightly" ]]; then
-      phpunit --configuration=phpunit.xml --include-path=lib/;
-      ~/.config/composer/vendor/bin/phpcs . --standard=PHPCompatibility --extensions=php -p --runtime-set testVersion 5.6-;
+  # Run unit tests on highest major version
+  - if [[ ${TRAVIS_PHP_VERSION:0:1} == "7" ]]; then
+      ~/.config/composer/vendor/bin/phpunit --configuration=phpunit.xml --include-path=lib/;
     fi
 
+php:
+  - 7.3
+
+env:
+  - PHPUNIT=6
+  - PHPUNIT=7
+  - PHPUNIT=8
+
 matrix:
   fast_finish: true
 
   include:
     - php: 5.6
+      env: PHPUNIT=
     - php: 7.0
-    - php: nightly
+    - php: 7.1
+    - php: 7.2
 
   allow_failures:
-    - php: nightly
+    - php: 7.3
+      env: PHPUNIT=7
+    - php: 7.3
+      env: PHPUNIT=8