provide ENVIRONMENT flag for Devel::Cover on 5.8.8

* DEVEL_COVER_UNSUPPORTED var must be set to run Devel::Cover
  on 5.8.8

* remove 5.8 from the array and explicitly add with env.
  * I couldn't get exclude: + include: to work to remove 5.8.8
* See the Travis Matrix documentation:
  https://docs.travis-ci.com/user/customizing-the-build

.travis.yml

@@ -16,7 +16,12 @@ perl:
 - '5.14'
 - '5.12'
 - '5.10'
-- '5.8'
+
+env:
+matrix:
+  include:
+  - perl: 5.8
+    env: DEVEL_COVER_UNSUPPORTED=1
 
 install:
 - cpanm --quiet --notest