Force posix style paths to avoid issues with backslashes on Windows

[MarioLiebisch]

By default, path.join() on Windows will use backslashes. While this works fine for file system access directly, it will break strings written to code files for obvious reasons (i.e. creating unintentional valid and invalid escape sequences), which then results in build errors, even for fresh templates:

Serving on http://localhost:4200/
The Broccoli Plugin: [RollupWithDependencies] failed with:
Error: <project-path>/tmp/rollup_with_dependencies-cache_path-sCSx6c9l.tmp/config/module-map.js: Bad character escape sequence (1:74)
    at error (<project-path>\node_modules\rollup\dist\rollup.js:170:12)
    at <project-path>\node_modules\rollup\dist\rollup.js:8937:6

The broccoli plugin was instantiated at:
    at RollupWithDependencies.Plugin (<project-path>\node_modules\broccoli-plugin\index.js:7:31)
    at RollupWithDependencies.Rollup (<project-path>\node_modules\broccoli-rollup\dist\index.js:74:72)
    at new RollupWithDependencies (<project-path>\node_modules\@glimmer\application-pipeline\dist\commonjs\es5\broccoli\rollup-with-dependencies.js:63:57)
    at GlimmerApp.rollupTree (<project-path>\node_modules\@glimmer\application-pipeline\dist\commonjs\es5\broccoli\glimmer-app.js:286:16)
    at GlimmerApp.javascriptTree (<project-path>\node_modules\@glimmer\application-pipeline\dist\commonjs\es5\broccoli\glimmer-app.js:267:21)
    at GlimmerApp.toTree (<project-path>\node_modules\@glimmer\application-pipeline\dist\commonjs\es5\broccoli\glimmer-app.js:212:27)
    at module.exports (<project-path>\ember-cli-build.js:23:14)
    at Builder.setupBroccoliBuilder (<project-path>\node_modules\ember-cli\lib\models\builder.js:54:19)
    at new Builder (<project-path>\node_modules\ember-cli\lib\models\builder.js:33:10)
    at ServeTask.run (<project-path>\node_modules\ember-cli\lib\tasks\serve.js:15:19)

[rwjblue]

Awesome, thank you!