Add temporary allow_failure for python 2.7/gcc 6 (#562)

Current debian testing has Python 2.7.13-RC1, which has a serious
regression (upstream https://bugs.python.org/issue5322) which should be
reverted for RC2 (or the final 2.7.13).  Ignore build failures for this
build test temporarily (with the intention of reverting this commit in a
couple of weeks once it stops failing, i.e. when debian testing picks up
an updated python 2.7 release).

.travis.yml

@@ -51,6 +51,7 @@ matrix:
     - flake8
   allow_failures:
     - env: PYTHON=3.5 CPP=17 GCC=7
+    - env: PYTHON=2.7 CPP=14 GCC=6 # Temporary until python >2.7.13-rc1 migrates to debian testing
 cache:
   directories:
   - $HOME/.cache/pip