Use travis_retry to avoid test nondterminism.

The end to end tests depend on a database, which means that
there's a big source of nondeterminism.

Even so, there's some problem with the test framework which
makes it much less deterministic on Travis than locally.

I can't log into a Travis server to diagnose there, so I'm
papering over this problem by using travis_retry as doced at:
docs.travis-ci.com/user/common-build-problems/#timeouts-installing-dependencies

.travis.yml

@@ -11,5 +11,5 @@ sudo: false
 addons:
   postgresql: "9.6"
 
-script:        ./scripts/validate.sh
-after_success: npm run coveralls
+script:        travis_retry ./scripts/validate.sh
+after_success: travis_retry npm run coveralls