Fix for Zabbix 3.4

[Menollo]

The SenderFrequency variable does not exist anymore in zabbix 3.4.

[bastelfreak]

Hi @Menollo, thanks for the PR. Can you take a look at the failling spec tests?

[Heidistein]

The failing tests are, as far as I understand, unrelated. Also older PR's suffer from this.
The failure is in:
Zabbix::Resources::Userparameters[foo_Zabbix_Proxy]

 Failure/Error: it { is_expected.to contain_zabbix__resources__userparameters('foo_Zabbix_Proxy') }

   expected that the catalogue would contain Zabbix::Resources::Userparameters[foo_Zabbix_Proxy]

 # ./spec/classes/proxy_spec.rb:109:in `block (5 levels) in <top (required)>'

[bastelfreak]

We fixed this in master. I rebased the branch. It should turn green now on travis.