Fix CI configuration after 4e8be41c.

.travis.yml

@@ -10,7 +10,7 @@ matrix:
     - rust: nightly
       env: FEATURES='std raw_socket tap_interface log' MODE='test'
     - rust: nightly
-      env: FEATURES='alloc collections' MODE='build'
+      env: FEATURES='alloc' MODE='build'
     - rust: nightly
       env: FEATURES='' MODE='build'
 script: