Add dash to JS build chain

Dash is now needed to run npm install.

Author: pbugnion

.travis.yml

@@ -5,6 +5,7 @@ cache:
 install:
   - "if [[ \"$GROUP\" == js ]] ; then npm -v ; fi"
   - "if [[ \"$GROUP\" == js ]] ; then npm install ; fi"
+  - "if [[ \"$GROUP\" == js ]] ; then pip install dash ; fi"
   - "if [[ \"$GROUP\" == python-linting ]] ; then pip install black flake8 isort ; fi"
 script:
   - "if [[ \"$GROUP\" == js ]] ; then npm run lint ; fi"