Change the security context of the script so logrotate can read it.

https://bugzilla.redhat.com/show_bug.cgi?id=1138477
Fixes #219

The shell script was inheriting a resource context of "httpd_sys_content_t" from the parent directory.
Logrotate runs as logrotate_t and was getting audit failures in the /var/log/audit/audit.log:

type=AVC msg=audit(1410073682.664:105): avc:  denied  { read } for  pid=9812 comm="sh" name="logrotate_free_space_check.sh" dev=dm-0 ino=548602 scontext=system_u:system_r:logrotate_t:s0-s0:c0.c1023 tcontext=system_u:object_r:httpd_sys_content_t:s0 tclass=file

[skip ci]