Remove the optional generic-array feature

- It can be added back later if needed
- A public dependency implies version coupling, and we can't afford to
  have it for a niche use case.

.travis.yml

@@ -5,7 +5,6 @@ matrix:
     - rust: 1.12.0
     - rust: stable
       env:
-        - FEATURES="use_generic_array"
         - NODEFAULT=1
     - rust: beta
     - rust: nightly
@@ -16,7 +15,7 @@ matrix:
       - NODROP_FEATURES='use_needs_drop'
     - rust: nightly
       env:
-      - FEATURES='use_union use_generic_array'
+      - FEATURES='use_union'
       - NODROP_FEATURES='use_union'
 branches:
   only: