Fix Infection job in travis under php7.3

Manually installs the latest copy of phpunit 8.x for php 7.3 because
while phpunit9 is compatible with php7.3, infection is not able to read
the new xml config schema that phpunit uses as of v9.

Author: dshoreman

.travis.yml

@@ -7,6 +7,7 @@ matrix:
       env: PHPUNIT_LEGACY="true"
     - php: 7.3
       dist: bionic
+      env: PHPUNIT_LEGACY="true"
     - php: 7.4
       dist: bionic
     - php: nightly
@@ -23,6 +24,18 @@ cache:
 install:
   - travis_retry composer install $COMPOSER_OPTS
 
+  # Where PHPUnit v8 is used, we need to replace the config.
+  - if [[ ${TRAVIS_PHP_VERSION:0:1} == "7" ]]
+    && [ ${TRAVIS_PHP_VERSION:2:1} -lt 4 ]; then
+      mv -v tests/phpunit.legacy.xml tests/phpunit.xml
+    ; fi
+
+  # PHPUnit 9 supports 7.3, but Infection PHP 18 can't read
+  # its config so instead we downgrade to PHPUnit 8.5 here.
+  - if [[ ${TRAVIS_PHP_VERSION:0:3} == "7.3" ]]; then
+      travis_retry composer require -W phpunit/phpunit:^8.5
+    ; fi
+
 script:
   - vendor/bin/grumphp run