travis: enable apt retry on fail

This is a workaround for an unsolved travis issue that is causing CI
instances to sporadically fail due to 'unable to connect' issues during
apt stage.

Ref: https://github.com/travis-ci/travis-ci/issues/8507
Ref: https://github.com/travis-ci/travis-ci/issues/9112#issuecomment-376305909

.travis.yml

@@ -2,6 +2,8 @@ language: c
 sudo: required
 addons:
     apt:
+        config:
+            retries: true
         sources:
             - ubuntu-toolchain-r-test
             - llvm-toolchain-precise-3.9