chore(travis): remove greenkeeper-lockfile script

Greenkeeper updates now the package-lock.json

.travis.yml

@@ -13,10 +13,3 @@ addons:
       - ubuntu-toolchain-r-test
     packages:
       - g++-4.9
-before_install:
-# package-lock.json was introduced in npm@5
-  - '[[ $(node -v) =~ ^v9.*$ ]] || npm install -g npm@latest' # skipped when using node 9
-  - npm install -g greenkeeper-lockfile
-install: npm install
-before_script: greenkeeper-lockfile-update
-after_script: greenkeeper-lockfile-upload