Allow Zabbix API v3.4 access (Closes: #81)

[ottok]

At least on our systems this version is now running in production and performing well.

[jrbeilke]

Need to update zabbixapi/spec/server.rb and it's high time we should drop support for 2.4 and 3.0 per the README (https://github.com/express42/zabbixapi#version-policy). I'll see what I can do.

it 'should be 2.4.x or 3.0.x or 3.2.x' do
  expect(zbx.server.version).to match(/(2\.4|3\.[02])\.\d+/)
end

[jrbeilke]

Looking over https://www.zabbix.com/life_cycle_and_release_policy and we may also want to consider changing the version policy for the gem to include the last two Zabbix versions along with the LTS release (ie. 3.2, 3.4, and 3.0 LTS).

[paskal]

From my experience working with Python library I can see what new API rarely have breaking changes: 3.2 library works with 3.4 and 4.0 as well without any changes. If your code raising proper errors in case API call returned an error, it seems safe not to check the version and assume the user will handle the problem.
It's a good practice not to hardcode supported versions list in my humble opinion.

[gdubicki]

Let's keep the current policy of official support for last 2 versions, so 3.4 and 4.0 for now.