travis output includes versioned list of installed packages / modules

Use the npm command for this since yarn does not currently limit
the output to only installed packages (it displays all packages).
The npm ls command seems to work fine even though we are trying
to use yarn here to actually manage the dependencies.

yarnpkg/yarn#3569

Author: danstoner

.travis.yml

@@ -4,7 +4,7 @@ services:
 install:
 - docker build -t $BUILDING .
 script:
-- docker run -v $TRAVIS_BUILD_DIR:/var/www/:ro -it $BUILDING yarn list
+- docker run -v $TRAVIS_BUILD_DIR:/var/www/:ro -it $BUILDING npm ls --depth 0
 - docker run -v $TRAVIS_BUILD_DIR/__tests__/mock:/var/www/__tests__/mock/:ro --rm
   -it $BUILDING yarn test
 after_success: