Use travis_retry to reduce spurious test failures (#3418)

.travis.yml

@@ -50,6 +50,6 @@ before_script:
   - ln -s /usr/bin/x86_64-linux-gnu-g++-6 "$HOME/g++"
 
 script:
-  - bundle exec parallel_test spec/ --group-by filesize --type rspec
+  - travis_retry bundle exec parallel_test spec/ --group-by filesize --type rspec
   - npm test
   - bundle exec i18n-tasks unused