Update peer dependencies to support ESLint v7

[IvanGoncharov]

Fixes warning I have during npm install:

warning " > eslint-plugin-istanbul@0.1.1" has incorrect peer dependency "eslint@^6.0.0".

[coreyfarrell]

Thank you for the contribution. Would you mind removing everything except the peerDependency update? This module still supports being tested on Node.js 8 so the devDependency updates cannot be merged yet. Dropping Node.js 8 support from istanbuljs modules is on hold until after Babel 8 is released (the plan is for our next semver-major to upgrade Babel).

[IvanGoncharov]

@coreyfarrell Done ✅

[coreyfarrell]

Thanks, this is merged and released as eslint-plugin-istanbul@0.1.2.