Fix release time for GitLab certification program

Handbook claims GitLab Certification Program (currently WIP) will be available Q2 2018

GitLab Certification Program currently is a work in progress, has not been released, and has no ETA set

MR updates

... will be rolling out a GitLab Certification Program in the second quarter of 2018.

to read:

... will be rolling out a GitLab Certification Program in the future.

Official issue:

• GitLab Cetification Progam Development

Related issue:

• Develop a GitLab Admin Certification