ci(travis): only install grpc once

grpc was being compiled twice when builds run on travis. This was causing
builds to take several minutes longer than they should. installation of
the dependencies listed in app/package.json happens automatically when
`electron-builder install-app-deps` runs as part of the postinstall step.

.travis.yml

@@ -31,7 +31,6 @@ addons:
 
 install:
   - yarn
-  - cd app && yarn && cd ..
   - "/sbin/start-stop-daemon --start --quiet --pidfile /tmp/custom_xvfb_99.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :99 -ac -screen 0 1280x1024x16"
 
 before_script:

appveyor.yml

@@ -25,7 +25,6 @@ install:
   - ps: Install-Product node $env:nodejs_version
   - set CI=true
   - yarn
-  - cd app && yarn
 
 test_script:
   - node --version