Add conditions for installing prometheus above version 2.0

[klatys]

I added few simple additions for installing version 2.0 and above.

[klatys]

about the failing build - not sure but is it possible this relates to containerd/containerd#337 somehow? not sure this is my mistake really..

[brainstorm]

@William-Yeh it would be rad to have this change merged into this repo, I'm looking into deploying prometheus 2.x... @klatys, is the Go environment correctly setup when running this on your machine or test setup?:

https://travis-ci.org/William-Yeh/ansible-prometheus/builds/302412553#L6986

[klatys]

OK, so I added test for v2 deploy, but have struggled with testing git deploys. At current versions master is broken for some systems - see builds here is it a bad idea to exclude testing git deploy? @William-Yeh

[klatys]

Alternatively, the master version could be added as allow_failures
https://docs.travis-ci.com/user/customizing-the-build/#Rows-that-are-Allowed-to-Fail
@William-Yeh