Move mutant into platform specific block

* Mutant currently only works under mri 19 and rbx, so specify that
  it only be installed within those environments.
Update mutant task to only run if mutant is available
Remove unnecessary bundle exec from the command

* If the task is being run under bundler, system calls should work
  the same as if they were run with bundle exec.
Remove unnecessary parenthesis from exception
Change system command to use an Array as input

* This causes the arguments to be escaped automatically, preventing
  shell injection (not a huge risk in this case, but may prevent
  some kinds of errors; I wish this was the default interface).