Make selenium logs writeable

It looks like on Yosemite when the log directories get created by launchd to handle stderr and stdout the directory ends up owned by root. Consequently, the Selenium service can't start because the current user can't write to the log files. So you want to put the log files in a directory that already exists and is writeable by the current user. `/usr/local/var/log` looks similar to what is done by the Solr forumla, so I went with that.
`#{var}` instead of `/usr/local/var`