[chore] Use helper plugin when running tests on Travis

This plugin allows us to test code related to plugins, like testing
issues that only happen when we have plugins using line attribs, for
example (see #3118).

.travis.yml

@@ -4,6 +4,7 @@ node_js:
 install:
   - "bin/installDeps.sh"
   - "export GIT_HASH=$(git rev-parse --verify --short HEAD)"
+  - "npm install ep_test_line_attrib"
 before_script:
   - "tests/frontend/travis/sauce_tunnel.sh"
 script: