CI: Add yarn into matrix.

[ryan-roemer]

alongside npm

Something like:

env:
  matrix:
    - PKG_TOOL=npm
    - PKG_TOOL=yarn

before_install:
  - '[ "$PKG_TOOL" != "npm" ] || npm install -g npm'
  - '[ "$PKG_TOOL" != "yarn" ] || npm install -g yarn'

install:
  - ${PKG_TOOL} install