pin json gem to version 2.7.1

* json gem v2.7.2 breaks ops-manifest unit tests

[#187373625](https://www.pivotaltracker.com/story/show/187373625)

Authored-by: Mark Stokan <stokanm@vmware.com>

internal/test/Dockerfile

@@ -40,5 +40,7 @@ WORKDIR /tmp/ops-manager/gems/ops-manifest
 RUN bundle install && \
   rm -rf *.gem && \
   bundle exec gem build ops-manifest.gemspec && \
-  gem install ops-manifest-*.gem --no-document
+  gem install ops-manifest-*.gem --no-document && \
+  gem uninstall json && \
+  gem install json --version 2.7.1
 RUN which ops-manifest