Support for querying customers and updating their emails

[jorgemanrubia]

This adds supports for two things:

• Passing params for listing operations. I wanted to be able to use query params when finding customers. I think supporting params in general for list requests makes sense.

• Support for customer email updates

It also includes some changes for making the travis build pass:

• Upgrades rake to deal with security advisory CVE-2020-8130 that was making Travis fail.
• Bump rubocop's ruby target version to make it work with version0.93 and make rubocop pass

Thanks for the nice lib!

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[waghanza]

I've made #25, maybe we can combine our efforts