Bump zabbixapi version to 4.2.0

[fraenki]

Pull Request (PR) description

This is a re-submission of #623, because the original PR was probably abandoned.

• Re-submission of Upgrade zabbixapi package for Zabbix 4.2 support #623 (with latest version of zabbixapi)
• Rebased against latest master branch
• Bumps acceptance tests to Zabbix LTS release 4.4 (it's too early for 5.x)

This Pull Request (PR) fixes the following issues

Fixes #621 (Zabbix 4.2 support)
Fixes #544 (Zabbix 3.4 support)
Fixes #689 (Zabbix 5.0 support; partially, should be considered the first step)

[fraenki]

@kenyon I've fixed the failing acceptance tests. Please merge :)

FWIW, Travis CI still reports a failure, but it's unrelated to my changes:

"error":"toomanyrequests: You have reached your pull rate limit. 
You may increase the limit by authenticating and upgrading: 
https://www.docker.com/increase-rate-limit"

[kenyon]

I reran the failed test, and it passed.

[bastelfreak]

thanks for the PR! I will do some updates to the others we've open and try to merge this one later today

[bastelfreak]

Related PRs:

• Drop CentOS 6 testing/support #733
• modulesync 4.0.0 #728

[fraenki]

@bastelfreak Thank you! I think it's safe to close #623 too (it is the abandoned PR).