Update .travis.yml

Author: Dave Conway-Jones

.travis.yml

@@ -21,6 +21,7 @@ cache:
 
 script:
   - |
+    npm i -g yarn
     if [ "$TRAVIS_OS_NAME" == "linux" ]; then
       docker run --rm \
         --env-file <(env | grep -vE '\r|\n' | grep -iE 'DEBUG|NODE_|ELECTRON_|YARN_|NPM_|CI|CIRCLE|TRAVIS_TAG|TRAVIS|TRAVIS_REPO_|TRAVIS_BUILD_|TRAVIS_BRANCH|TRAVIS_PULL_REQUEST_|APPVEYOR_|CSC_|GH_|GITHUB_|BT_|AWS_|STRIP|BUILD_') \
@@ -37,4 +38,4 @@ before_cache:
 
 branches:
   except:
-    - "/^v\\d+\\.\\d+\\.\\d+$/"
+    - "/^v\\d+\\.\\d+\\.\\d+$/"