Update slowfs_cache third-party module from 1.7 to 1.8.
Do not bump PORTREVISIONs because this module disabled by default.
2012-02-28 VERSION 1.8
* Fix setting of slowfs process title.
In case when local path was over 277 characters long, slowfs
process would crash, which would result in file not being
copied to the cache.
Bug had appeared in version 1.7.