Upgrade mutant dependency to 0.8.9

[backus]

• Changes mutant task to conform with changes to return value of Mutant::CLI.run
• Adds check for circle ci in mutant rake
  • Mutant is currently defaulting to 32 jobs on circle. When on circle ci this should be 4 jobs. Mutant used to handle this until 0.8.9

[backus]

I don't know what the deal is with the build on circle tripping on a bunch of neutral failures. I reran it with SSH enabled and pulled down the Gemfile.lock to see if it might be a new dependency my local build didn't use. Still passes on my machine.

Edit: Issue was circle running mutant with 32 threads.

[backus]

@dkubb Build is green if you want to take another look

[backus]

@dkubb updated once more!