Used type Integer instead of Fixnum (avoid warning since ruby 2.4)

[tirolerstefan]

Avoid a warning that is triggered in util.rb with ruby 2.5: /usr/lib/ruby/vendor_ruby/main/util.rb:47: warning: constant ::Fixnum is deprecated

In https://github.com/sass/sass/issues/2218 I found a discussion whether they tested the replacement to "Integer" and it worked in all ruby versions back to 1.8.7.

Feel free to test it.