Added gridtk in the requirements [skip ci]

3 jobs for master in 17 minutes and 9 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #154546
docker
build_linux_36

00:16:42

passed #154547
macosx
build_macosx_36

00:02:03

 
  Deploy
passed #154548
deployer
deploy_beta

00:00:26