Move PHP 5.4 tests from WP_VERSION latest to 5.1

Related wp-cli/wp-cli#5127

Author: wojsmol

.travis.yml

@@ -71,4 +71,4 @@ jobs:
     - stage: test
       php: 5.4
       dist: precise
-      env: WP_VERSION=latest
+      env: WP_VERSION=5.1