use Fermat's little theorem instead of Wilson's theorem for deciding if a number is prime or not

Fermat's little theorem provides faster results than Wilson's theorem most of all because it doesn't use factorials. The factorial calculation is no longer necessary, so I'm moving the factorial to attic.
ignore slow test cases when running spec, but let them run on CI

travis CI will call rake so I made a Rakefile that will run all test cases by default, but I'm ignoring slow test cases when running rspec