Add support for Ruby 2.7

[nbulaj]

Add support for Ruby 2.7 both in Travis and in Grape itself

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Ruby 2.7.0 was released December 25, 2019, and has some noisy deprecation messages about wrong usage of keyword arguments.

Grape gem already correctly uses double-splat operator in some places, but not all over the code-base. That must be fixed.

So, what's done here:

• Add Ruby 2.7 to Travis matrix, use 2.6.x for Gemfile and rails 5 as it's already done for previous versions.
• Fix method signatures and calls that produce deprecation messages for kwargs used.

To see a ton of deprecation messages you can just install Ruby 2.7, checkout current master branch and run test suite. There are still some messages not related to the gem itself that comes from mustermann, multi_json and a few other gems that must be fixed themselves.

[anatol]

Is there any plan to release a gem version that supports ruby2.7 (i.e. with the ruby27 fixes above)?

[dblock]

Is there any plan to release a gem version that supports ruby2.7 (i.e. with the ruby27 fixes above)?

Yes, but at the very least we need to check #1957. Maybe you can help?

[dblock]

#1958