Shallow-clone WP for testing

Props @osiux

Author: rmccue

.travis.yml

@@ -12,7 +12,7 @@ php:
 # Clones WordPress and configures our testing environment.
 before_script:
     - export PLUGIN_SLUG=$(basename $(pwd))
-    - git clone git://develop.git.wordpress.org/ /tmp/wordpress
+    - git clone --depth=1 git://develop.git.wordpress.org/ /tmp/wordpress
     - mkdir "/tmp/wordpress/src/wp-content/plugins/$PLUGIN_SLUG"
     - cp -r . "/tmp/wordpress/src/wp-content/plugins/$PLUGIN_SLUG/"
     - cd /tmp/wordpress