Update rspec matcher to work with custom version association names

`have_a_version_with` and `have_a_version_with_changes` had the
`versions` association call hardcoded.
If `has_paper_trail` had the `versions` option set to something else
(e.g: `has_paper_trail versions: :paper_trail_versions`) the matcher
couldn't be used.

This fixes that issue by dynamically fetching the association name from
the `:versions_association_name` class attribute.

lib/paper_trail/frameworks/rspec.rb

@@ -25,10 +25,16 @@
 
 RSpec::Matchers.define :have_a_version_with do |attributes|
   # check if the model has a version with the specified attributes
-  match { |actual| actual.versions.where_object(attributes).any? }
+  match do |actual|
+    versions_association = actual.class.versions_association_name
+    actual.send(versions_association).where_object(attributes).any?
+  end
 end
 
 RSpec::Matchers.define :have_a_version_with_changes do |attributes|
   # check if the model has a version changes with the specified attributes
-  match { |actual| actual.versions.where_object_changes(attributes).any? }
+  match do |actual|
+    versions_association = actual.class.versions_association_name
+    actual.send(versions_association).where_object_changes(attributes).any?
+  end
 end