Ensure test for rails 5.2

[waghanza]

Hi,

This PR resolve #623

It add testing for Rails 5.2 on CI

Regards,

PS : May I suggest to fix constraint to dynamoid ~> 1.3 for rails 5.0 and 5.1
Support was added from this version @see https://github.com/Dynamoid/Dynamoid/blob/master/CHANGELOG.md#130

[anilmaurya]

Thank you @waghanza for contributing. I appreciate your help 👍
Regarding fix constraint to dynamoid ~> 1.3 for rails 5.0 and 5.1, please raise a separate PR for it.