ci: use npm to install greenkeeper

Author: SimenB

.travis.yml

@@ -8,7 +8,7 @@ os:
 - linux
 - osx
 osx_image: xcode7.3
-before_install: yarn global add greenkeeper-lockfile@1
+before_install: npm install --global greenkeeper-lockfile@1
 before_script: greenkeeper-lockfile-update
 script: yarn run travis
 after_script: greenkeeper-lockfile-upload