Don't install proxy_html package in ubuntu xenial

[amateo]

In Ubuntu Xenial, proxy_html module is include in apache standard
packages, so don't try to install it

[eputnam]

@amateo thanks for your work here! Do you know if the same is true for Debian 9? If not, please adjust the conditional accordingly.

[mmoll]

It's also included in apache2-bin on Debian 9:

$ cat /etc/debian_version
9.0
$ dpkg -S /usr/lib/apache2/modules/mod_proxy_html.so
apache2-bin: /usr/lib/apache2/modules/mod_proxy_html.so

[eputnam]

ugh, i read the pr wrong. thanks @amateo @mmoll !