Modify cellminer.rb to run without dependence of midstates

It might be a little more efficient, also the option on slush's miner proxy of -nm aka no midstates breaks the cellminer from running correctly.

Do you guys think it would be possible to add a line to handle a null midstate?

Thanks!

[2014-02-22 22:14:38] Cell Miner starting [2014-02-22 22:14:38] Creating 6 SPU miner(s) [2014-02-22 22:14:38] Creating 2 PPU miner(s) [2014-02-22 22:14:38] Starting long poll [2014-02-22 22:14:38] Got work... 0.000 Mhash/s, 0 backlogged work items target = 00000000ffff0000000000000000000000000000000000000000000000000000 prev = 00000000000000015332feef580c1eed2f7da1e4e2ad8cd57ff0b735606b1526 /home/sigkill/src/cellminer/cellminer.rb:144:in run': can't convert nil into String (TypeError) from /home/sigkill/src/cellminer/cellminer.rb:144:inblock (2 levels) in main' from /home/sigkill/src/cellminer/cellminer.rb:138:in loop' from /home/sigkill/src/cellminer/cellminer.rb:138:inblock in main'