build fixes (#362)

* add production web build to travis tests

* remove old builds before starting new build

.travis.yml

@@ -14,5 +14,6 @@ install:
 script:
   - set -eo pipefail
   - flake8 server/app/
+  - npm run build
   - npm run --prefix client/ test
   - pytest -s server/test

bin/build-client

@@ -6,6 +6,7 @@ cd $CELLXGENE_DIR
 
 npm install --prefix client/ client
 npm run  --prefix client build
+rm -rf server/app/web/static
 mkdir -p server/app/web/static/img
 cp client/build/index.html server/app/web/templates/
 cp -r client/build/static server/app/web/