Add 5-2-stable Ruby 2.2 build to allowed failures

There's a minor difference:

    Widget management
      User creates a new widget

    -1 example, 0 failures, 1 pending
    +1 example, 0 failures

most probably caused by 5-2-stable vs 5.2.4.x spec generation
difference.

Author: pirj

.travis.yml

@@ -212,6 +212,11 @@ matrix:
       env: RAILS_VERSION=master
     - rvm: 2.4.10
       env: RAILS_VERSION=master
+    - rvm: 2.2.10
+      # An actual incompatibility of 5.2.4.0-5.2.4.3 forces to use `5-2-stable`,
+      # but there's a minor difference in the generated tests that results in a
+      # minor failure.
+      env: RAILS_VERSION='5-2-stable'
   fast_finish: true
 
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