Fix warnings about `Fixnum` in tests
Fix warnings about `Fixnum` in tests

When running the tests under Ruby 2.4.0+, the following warning is
printed a few times:

    spec/support/matchers.rb:73: warning: constant ::Fixnum is deprecated

`Fixnum` was deprecated in Ruby 2.4.0, but `Numeric` exists in all Ruby
versions that Travis tests against.

bogdan/datagrid,bogdan/datagrid