travis-ci: Remove clang 4/c++17 from allow_failures

The job is using the released clang and stable-branch libc++, which
wasn't the case when it was added.  Leave the g++7/c++17 in
allow_failures for now as it's still a pre-release compiler (and pulled
from debian experimental).

.travis.yml

@@ -78,7 +78,6 @@ matrix:
     - flake8
   allow_failures:
     - env: PYTHON=3.5 CPP=17 GCC=7
-    - env: PYTHON=3.6 CPP=17 CLANG=4.0
 cache:
   directories:
   - $HOME/.local/bin