Deprecate Puppet 3 as supported platform

[lazyfrosch]

Requirement for #350 / #345

Removing Puppet 3 support for puppet-icinga2.

@lbetz @bobapple Can I get your opinion on that.

Strategy:

• Creating a puppet3 branch, which won't be maintained, but kept as reference
• Removing travis tests for Puppet 3
• Updating metadata accordingly
• Doing next release as 1.4.0 - upstream deprecation shouldn't be our breaking release...

[bobapple]

I don't think we should try to maintain a separate branch for Puppet 3 support. My experience tells me that it will be a dead branch without any maintenance or releases.

I would prefer a 2.0 of this module where we can change many things that break compatibility. Dropping Puppet 3 support would be one of the tasks. Additionally, breaking compatibility in v1.4 breaks conformity with SemVer, which I'm trying to follow.

As always, time is a matter. I don't think we should work on a v2.0 before January 2018. There are many issues and PRs open currently that need to be reviewed, merged, cleaned up. That is what I think is the primary goal for the next weeks.

Please refer to this puppet-icingaweb2 issue regarding this topic: voxpupuli/puppet-icingaweb2#164

[lazyfrosch]

@bobapple I agree, just tell me what I should take care of...

[lbetz]

Will be merged to the next Version, 2.0.