Minor cleanup of the Rakefile

What the title says, doing things a bit more properly

* use `Pathname#dirname` rather than `join('..')`, 
* remove unused BENCH_ROOT
* ruby 1.9 doesn't support LoadError#path, use message
* $stderr instead of STDERR
* use proper exit code (man sysexits)

Rather than catching that LoadError though, wouldn't it be more appropriate to just stick benchmark-ips in the gemspec as a development dependency?