Fix disk capacity reporting for Mac OS X

Hey Derek,

this basically just introduces some error handling when /proc/diskstats is missing, so that the disk capacity will be reported on OS X (Disk I/O is still missing).

I also had to add some RSpec examples since existing tests blew up on OS X (probably due to missing stuff below `/proc`); you can run them with `bundle exec rspec` after installing bundles gems.
Feel free to remove those from my pull request if you don't wanna mess with another testing framework.
The gem version was incremented as well.


Björn