Ruby 2.4 unifies Fixnum and Bignum into Integer

For now, make the test expectations different for ruby 2.4.0+ vs. prior rubies.

CustomAttribute#value_type returns a symbol for a ruby core class, a
hardcoded value instead of the class's behavior.
Do we use this?  Why are we returning a value instead of caring about
behavior of the old Fixnum and Integer?  If we don't use this, we
should remove the whole method and it's tests added in:
4687394

The PR that added this was:
#11000

Adding CustomAttribute to provider UI/API was added in:
#10897

spec/models/custom_attribute_spec.rb

@@ -12,6 +12,9 @@
   end
 
   it "returns the value type of Fixnum custom attributes" do
-    expect(int_custom_attribute.value_type).to eq(:fixnum)
+    # TODO: Ruby 2.4 unifies fixnum and bignum into integer, we shouldn't be
+    # returnings ruby types like this.
+    expected = RUBY_VERSION >= "2.4.0" ? :integer : :fixnum
+    expect(int_custom_attribute.value_type).to eq(expected)
   end
 end