Fix for stale PIDs not being cleared properly

If you have a stale PID in the file that is longer than the new PID the
tail of the old PID will be left in the PID file.

eg - old PID is NNNN, new pid is PPP.  Final PID in file is PPPN.
Update filesystem spec to work against ruby <1.9.3
Pidfile bad if pidfile is stale for other process

If PID in the pidfile is found running, and the file is not locked, the
old PID and new PID will be merged.

eg - old PID is NNNN, new pid is PPP.  Final PID in file is NNNNPPP.
Update filesystem spec to work against ruby <1.9.3
Add 2.0 and 2.1 versions of ruby for Travis
Fixed warning when running with ruby -w
Merge pull request #4 from whitty/master

Fixes for incorrect PIDs written into stale PID files 
@whitty Thanks for the contribution, I'm going to release it again immediately.