Include ohai

build chefdk with ohai @opscode/client-eng 
looks like its just for a performance boost to bignums.  it was introduced last year.  disabling gmp will reduce the GPL footprint in our codebase.

i think i'm fine with it.

https://groups.google.com/forum/#!msg/ruby-core-google/R6L7kARe6cM/U4W3nRz_lXsJ