Lock down nodejs version

The new Travis Trusty image uses node 8 (latest LTS) as the default.
Node 8 is incompatible with some modules used for the UI.

.travis.yml

@@ -43,6 +43,7 @@ before_install:
 install:
   - if [[ -z "$SKIP_NOMAD_TESTS" ]]; then make deps ; fi
   - if [[ "$RUN_STATIC_CHECKS" ]]; then make lint-deps ; fi
+  - if [[ "$RUN_UI_TESTS" ]]; then . $HOME/.nvm/nvm.sh && cd ui && nvm use && cd .. ; fi
 
 script:
   - sudo -E "PATH=$PATH" make travis

ui/.nvmrc

@@ -0,0 +1 @@
+6