Repeated whitespace causes strange parsing of fractions


    Fractional.to_f("6 5/8")   # => 6.625
    Fractional.to_f("6  5/8")  # => -0.625

I had to upgrade a few dependencies to get specs working. Let me know if you'd prefer that as a separate pull request. (Also, there appears to be an existing failure at `spec/fractional_class_methods_spec.rb:70`.)