Update .travis.yml

Author: GrahamCampbell

.travis.yml

@@ -23,9 +23,9 @@ matrix:
     - php: nightly
       dist: bionic
   fast_finish: true
-  allow_failures:
-    - php: nightly
 
-install: travis_retry composer update
+install:
+  - if [ "$TRAVIS_PHP_VERSION" != "nightly" ]; then travis_retry composer update; fi
+  - if [ "$TRAVIS_PHP_VERSION" == "nightly" ]; then travis_retry composer update --ignore-platform-reqs; fi
 
 script: vendor/bin/phpunit