Support `--server apache2`

[Jeroen Wiert Pluimers]

Despite the --help indicating --server apache2 is supported, it isn't.

Will submit a pull-request shortly.

Help

# ./checkssl --help
checkssl ver. 1.21
Checks ssl certs for a set of domains

Usage: checkssl [-h|--help] [-d|--debug] [-f|--file filename] [-s|--server stype] [-l|--location directory] 
                [-e|--expires days] [-r|--renew] [-u|--update] [-U|--nocheck] [-c|--command command] [domain]

Options:
  -h, --help      Display this help message and exit.
  -d, --debug     Outputs debug information
  -f, --file  filename
                  Where 'filename' is a file containing a list of domain names
  -s, --server server_type
                  Where 'server_type' is the server type (cpanel, ISPconfig, apache2 ...)
  -l, --location directory
                  Where 'directory' is where your lets encrypt live directory is
                  (typically /etc/letsencrypt/live/)
  -e, --expires days
                  Where 'days' is the number of days to alert if cert expires in that time period
  -r, --renew     This just lists domain names that need to be renewed.
                  This list could be used by an auto renew script, or to email you.
  -p, --problems  This just lists the domains that have possible issues.
                  This list could be used to email you only if there is something to take care of.
  -u, --upgrade   Upgrade checkssl if a more recent version is available
  -U, --nocheck   Do not check if a more recent version is available
  -c, --command run_command
                  Where 'run_command' is a command which will be run (with domain name passed)
                  for any certs due for renewal

                  A domain name can also be specified on the command line

Error

# ./checkssl --server apache2
unknown server type currently

[Andy - Serverco]

Fixed via your PR - Thanks :)