Avoid usage of deprecated Fixnum class

Aggelos Avgerinosrequested to merge
avgerinos/gitlab-grit:drop-fixnum into master

As of Ruby 2.4 Fixnum is deprecated due to the Integer class unification1 and instead the suggested class to use is Integer.

Since Fixnum is an Integer we can safely use Integer regardless of the ruby version.

Fixnum.ancestors.include?(Integer)
=> true

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