Re-install test/dev dependencies before running tests

before_script used to trigger “yarn install --no-progress --production”,
which would uninstall dependencies required to run the tests.

Until railties is fixed or we move to Circle CI, work around that
issue by re-installing dev dependencies.

.travis.yml

@@ -53,6 +53,7 @@ before_install:
 
 before_script:
   - travis_wait ./bin/rails parallel:create parallel:load_schema parallel:prepare assets:precompile
+  - yarn install --no-progress
 
 script:
   - travis_retry bundle exec parallel_test spec/ --group-by filesize --type rspec