Merge branch '0.4' of https://github.com/bluss/arrayvec into merge-0.4

* '0.4' of https://github.com/bluss/arrayvec:
  0.4.9
  TEST: Add test that ensures the MaybeUninit impl is used on nightly
  FIX: Remove use of uninitialized in ArrayString
  FEAT: Implement a "MaybeUninit" and use it conditionally
  TEST: Add test that Some(ArrayVec<[&_;_]>).is_some()
  MAINT: Test the 0.4 branch in travis

.travis.yml

@@ -19,13 +19,16 @@ matrix:
     - rust: nightly
       env:
         - NODEFAULT=1
+        - ARRAYVECTEST_ENSURE_UNION=1
     - rust: nightly
       env:
       - NODROP_FEATURES='use_needs_drop'
+      - ARRAYVECTEST_ENSURE_UNION=1
     - rust: nightly
       env:
       - FEATURES='serde use_union'
       - NODROP_FEATURES='use_union'
+      - ARRAYVECTEST_ENSURE_UNION=1
 branches:
   only:
     - master