Require Xenial for builds
Require Xenial for builds

An attempt to fix the following error:
```
$ git clone https://git.zabbix.com/scm/zbx/zabbix.git --depth 1 --single-branch --branch $BRANCH
Cloning into 'zabbix'...
fatal: unable to access 'https://git.zabbix.com/scm/zbx/zabbix.git/': gnutls_handshake() failed: Handshake failed
The command "git clone https://git.zabbix.com/scm/zbx/zabbix.git --depth 1 --single-branch --branch $BRANCH" failed and exited with 128 during .
```
i-ky/zaipcs