travis: try some older WP versions

.travis.yml

@@ -2,15 +2,12 @@ language: php
 
 php:
     - 5.3
-    - 5.4
     - 5.5
-    - 5.6
-    - hhvm
 
 env:
-    - WP_VERSION=3.7
-    - WP_VERSION=3.8
-    - WP_VERSION=latest
+    - WP_VERSION=3.0.6
+    - WP_VERSION=3.5.2
+    - WP_VERSION=3.6.1
 
 before_script:
     - bash bin/install-wp-tests.sh wordpress_test root '' localhost $WP_VERSION