Add 7.2 with prefer lowest to ensure httplug 1.0 compatiblity

Author: joelwurtz

.travis.yml

@@ -24,6 +24,8 @@ matrix:
     include:
         - php: 7.1
           env: COMPOSER_FLAGS="--prefer-stable --prefer-lowest" COVERAGE=true TEST_COMMAND="composer test-ci"
+       -  php: 7.2
+          env: COMPOSER_FLAGS="--prefer-stable --prefer-lowest"
 
 before_install:
     - travis_retry composer self-update