nebular: Nested mixins error

Issue type

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• [ x ] bug report
• feature request

Issue description

Current behavior: Error about nested mixins when building/running (yarn run start)

ERROR in ./node_modules/raw-loader!./node_modules/postcss-loader/lib??embedded!./node_modules/sass-loader/lib/loader.js??ref--8-3!./src/app/@theme/styles/styles.scss
Module build failed: 
  @mixin hover-border($color) {
        ^
      Mixins may not be defined within control directives or other mixins.
      in /home/richard/workspace/ngx-admin/node_modules/@nebular/theme/components/checkbox/_checkbox.component.theme.scss (line 8, column 10)

Expected behavior: It should build and run.

Steps to reproduce: Clone from git and yarn run start

Other information:

npm, node, OS, Browser Yarn version 1.3.2 (also tried npm 5.6.0) Node 9.8.0 Arch Linux x86

Angular, Nebular

    "@angular/animations": "~5.2.6",
    "@angular/common": "~5.2.6",
    "@angular/compiler": "~5.2.6",
    "@angular/core": "~5.2.6",
    "@angular/forms": "~5.2.6",
    "@angular/http": "~5.2.6",
    "@angular/platform-browser": "~5.2.6",
    "@angular/platform-browser-dynamic": "~5.2.6",
    "@angular/router": "~5.2.6",
...
    "@nebular/auth": "2.0.0-rc.6",
    "@nebular/theme": "2.0.0-rc.6",
    "@nebular/security": "2.0.0-rc.6",