Need to add minitest to Gemfile

[Crag Wolfe]

conductor/src/Gemfile currently does not have a dependency for minitest, 
but it should.  I ran into this while trying to run "bundle exec rake 
spec" and "bundle exec rake cucumber" in a bundler-development 
environment (thanks Richard for helping me debug this).

I'm not certain whether this dependency belongs in the top section as 
"gem 'minitest' (which works fine for me) or just under "group 
:development, :test do"

--Crag

P.S. Is there a reason we don't have the lists of gems in Gemfile 
alphabetized? :-)