Fix RuboCop issues

[tatsuyafw]

Hi!

This PR contains the following fixes for RuboCop:

• Follow up RuboCop v0.77
  • The Layout/IndentFirstHashElement cop has renamed to Layout/FirstHashElementIndentation (Further cop name standardisation rubocop/rubocop#7468).
• Remove the unnecessary disabling of Style/IfUnlessModifier cop to resolve the failed CI:
  • https://travis-ci.org/ruby-grape/grape-entity/jobs/606754710

[grape-bot]

	1 Error
🚫	One of the lines below found in CHANGELOG.md doesn’t match the expected format. Please make it look like the other lines, pay attention to periods and spaces.

	2 Warnings
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.
⚠️	There’re library changes, but not tests. That’s OK as long as you’re refactoring existing code.

* [#307](https://github.com/ruby-grape/grape-entity/pull/307): Allow exposures to call methods defined in modules included in an entity [@robertoz-01](https://github.com/robertoz-01).

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

* [#326](https://github.com/ruby-grape/grape-entity/pull/326): Fix rubocop issues - [@tatsuyafw](https://github.com/tatsuyafw).

Generated by 🚫 danger

[coveralls]

Coverage increased (+0.01%) to 95.864% when pulling adc29e1 on tatsuyafw:rubocop-fixes into 1515b25 on ruby-grape:master.

[LeFnord]

thanks @tatsuyafw