chore(travis): install yarn via 'npm i -g yarn' to get latest version.

Do not cache .downstream_projects

.travis.yml

@@ -4,8 +4,7 @@ services:
   - xvfb
 before_install:
   - sudo apt-get install libgconf-2-4
-  - export PATH=$HOME/.yarn/bin:$PATH
-  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.5.1
+  - npm install -g yarn
 script:
   - tsc &&
     yarn test &&
@@ -23,4 +22,3 @@ git:
 cache:
   directories:
     - node_modules
-    - '.downstream_cache'