Avoid warning messages of squid

There were these error messages in /var/webmin/miniserv.error when squid is not installed.

Use of uninitialized value $squid::squid_version in substitution (s///) at ./squid-lib.pl line 20.
Use of uninitialized value $squid::squid_version in numeric ge (>=) at ./squid-lib.pl line 295.
Use of uninitialized value $squid::squid_version in numeric ge (>=) at ./squid-lib.pl line 302.
Use of uninitialized value $squid::squid_version in numeric ge (>=) at ./squid-lib.pl line 307.
Use of uninitialized value $squid::squid_version in numeric ge (>=) at ./squid-lib.pl line 312.
Use of uninitialized value $squid::squid_version in numeric ge (>=) at ./squid-lib.pl line 316.
Merge pull request #120 from yukoba/master

Avoid warning messages of squid