Update rubocop gem to version ~> 0.68.1

[fotos]

Fixes build by updating and locking rubocop gem to version ~> 0.68.1.

Version 0.68.1 is the last version that supports Ruby 2.2, since version 0.69.0 dropped Ruby 2.2 support in rubocop/rubocop#6945.

Additionally Layout/IndentHash has been renamed to IndentFirstHashElement in 0.68.0.

We should probably consider dropping support for Ruby 2.2, in a separate MR, since it has reached EOL since March 2018.

[grape-bot]

	1 Warning
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#41](https://github.com/ruby-grape/grape-swagger-entity/pull/41): Update rubocop gem to version ~> 0.68.1 - [@fotos](https://github.com/fotos).

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[LeFnord]

thanks @fotos …

yeap, support for ruby 2.2 will be droped in one of the next releases

[kzaitsev]

@fotos Hello, thanks for your contribution

[kzaitsev]

@LeFnord Hi Peter, do you have a plan to release grape-swagger 0.34.0?

After #40 I have a plan to release 0.3.4, as the last version which supports ruby 2.2.x and 2.3.x ( and add a new rubies to CI ).

I want to lock next releases on 0.4.x to grape-swagger 0.34.0 and greater.

[LeFnord]

Yeah the next release is comming, after this grape-swagger#738 is mörged, this will be the last one with supprt for grape < 1.3.0

after this is done, I will prepare the 1.0.0, which supports grape >= 1.3.0, have to do it so, cause of breaking changes (virtus vs. dry-types) and will be released after grape 1.3.0 was