Force bundler to use single thread for building Ruby gems.
Force bundler to use single thread for building Ruby gems.

This is due to a bug, see: https://github.com/rubygems/rubygems/pull/1005

Signed-off-by: Krzysztof Wilczynski <9bf091559fc98493329f7d619638c79e91ccf029@linux.com>

kwilczynski/ruby-fizzbuzz,kwilczynski/ruby-fizzbuzz,kwilczynski/ruby-fizzbuzz