Prevent grandchild from crashing on exception

execute_blocking's grandchild can have an exception thrown after cmd_err_write.close, in which case the rescue block will crash.
Use Process::exit! refs \#1
Mention about license in gemspec, refs arya/bluepill#197
Use blue-daemons fork of daemons gem, refs #1
Version bump to 0.0.67
update rspec syntax from should to expect
Update rspec dependency to >= 3