Add dash to JS build chain

Dash is now needed to run npm install.

Author: pbugnion

.travis.yml

@@ -3,6 +3,7 @@ cache:
   directories:
     - "node_modules"
 install:
+  - "if [[ \"$GROUP\" == js ]] ; then pip install dash ; fi"
   - "if [[ \"$GROUP\" == js ]] ; then npm -v ; fi"
   - "if [[ \"$GROUP\" == js ]] ; then npm install ; fi"
   - "if [[ \"$GROUP\" == python-linting ]] ; then pip install black flake8 isort ; fi"