use more specific version for lowest build to avoid timeout during installation

Author: dbu

.travis.yml

@@ -39,7 +39,7 @@ matrix:
     - php: 7.2
       env: STABILITY=dev SYMFONY_VERSION=4.0.*
     - php: 7.1
-      env: STABILITY=dev COMPOSER_FLAGS="--prefer-lowest" SYMFONY_VERSION=2.8.* SYMFONY_DEPRECATIONS_HELPER=weak
+      env: STABILITY=dev COMPOSER_FLAGS="--prefer-lowest" SYMFONY_VERSION=2.8.0 SYMFONY_DEPRECATIONS_HELPER=weak
     - php: 7.2
       env: STABILITY=dev SYMFONY_VERSION=3.3.*
     - php: 7.2