stopgap: allow travis to fail for Python 2.6 again

- Travis still fails on `spack spec` in Python 2.6.
- Allow the failure until we get to the bottom of it.

.travis.yml

@@ -112,6 +112,8 @@ jobs:
       env: TEST_SUITE=docker
   allow_failures:
     - env: TEST_SUITE=docker
+    # temporary Python 2.6 exception while we figure out why Travis is hanging
+    - python: '2.6'
 
 stages:
   - 'style checks'