Avoid Fixnum deprecation warnings in ruby 2.4

[jhawthorn]

In ruby 2.4, Fixnum and Bignum have been merged into a unified Integer class. In previous versions there should be no harm in treating them the same (both were Integer subclasses), so we might as well just check for Integer.

Fixes

versioncake-3.2.0/lib/versioncake/strategies/extraction_strategy.rb:8:
  warning: constant ::Fixnum is deprecated

[coveralls]

Coverage remained the same at 100.0% when pulling 8aed72c on jhawthorn:ruby_2_4_deprecation_warnings into 7c1d9d2 on bwillis:master.

[bwillis]

Awesome looks good, thanks! 🌈

[joshhepworth]

Any estimate on when this will be available on rubygems?