Plugin install fails with an optimizations error in 5.2-RC1

[tbragin]

Kibana/ES version:
5.2-RC1 internal build

Server/Browser OS version:
OSX 10.11.6 (15G31)

Original install method (e.g. download page, yum, from source, etc.):
Manual

Description of the problem including expected versus actual behavior:
On install, xpack fails with error below

tatianium:kibana-5.2.0-darwin-x86_64 tanya$ bin/kibana-plugin install file:///Users/tanya/x-pack-5.2.0-RC1.zip
Attempting to transfer from file:///Users/tanya/x-pack-5.2.0-RC1.zip
Transferring 124529944 bytes....................
Transfer complete
Retrieving metadata from plugin archive
Extracting plugin archive
Extraction complete
Optimizing and caching browser bundles...
Plugin installation was unsuccessful due to error "Optimizations failure.

    ERROR in ./src/ui_framework/components/index.scss
    Module build failed: Error: Missing binding /Users/tanya/kibana-5.2.0-darwin-x86_64/node_modules/node-sass/vendor/darwin-x64-48/binding.node
    Node Sass could not find a binding for your current environment: OS X 64-bit with Node.js 6.x
    
    Found bindings for the following environments:
      - Linux 64-bit with Node.js 6.x
    
    This usually happens because your environment has changed since running `npm install`.
    Run `npm rebuild node-sass` to build the binding for your current environment.
        at Object.<anonymous> (/Users/tanya/kibana-5.2.0-darwin-x86_64/node_modules/node-sass/lib/index.js:15:11)
        at Module._compile (module.js:570:32)
        at Module._extensions..js (module.js:579:10)
        at Object.require.extensions.(anonymous function) [as .js] (/Users/tanya/kibana-5.2.0-darwin-x86_64/node_modules/babel-core/lib/api/register/node.js:214:7)
        at Module.load (module.js:487:32)
        at tryModuleLoad (module.js:446:12)
        at Function.Module._load (module.js:438:3)
        at Module.require (module.js:497:17)
        at require (internal/module.js:20:19)
        at Object.<anonymous> (/Users/tanya/kibana-5.2.0-darwin-x86_64/node_modules/sass-loader/index.js:4:12)
        at Module._compile (module.js:570:32)
        at Module._extensions..js (module.js:579:10)
        at Object.require.extensions.(anonymous function) [as .js] (/Users/tanya/kibana-5.2.0-darwin-x86_64/node_modules/babel-core/lib/api/register/node.js:214:7)
        at Module.load (module.js:487:32)
        at tryModuleLoad (module.js:446:12)
        at Function.Module._load (module.js:438:3)
        at Module.require (module.js:497:17)
        at require (internal/module.js:20:19)
        at DependenciesBlock.loadPitch (/Users/tanya/kibana-5.2.0-darwin-x86_64/node_modules/webpack-core/lib/NormalModuleMixin.js:193:17)
        at DependenciesBlock.loadPitch (/Users/tanya/kibana-5.2.0-darwin-x86_64/node_modules/webpack-core/lib/NormalModuleMixin.js:186:21)
        at DependenciesBlock.loadPitch (/Users/tanya/kibana-5.2.0-darwin-x86_64/node_modules/webpack-core/lib/NormalModuleMixin.js:223:61)
        at DependenciesBlock.loadPitch (/Users/tanya/kibana-5.2.0-darwin-x86_64/node_modules/webpack-core/lib/NormalModuleMixin.js:186:21)
     @ ./src/ui/public/autoload/styles.js 4:0-54
"

cc: @tylersmalley @cjcenizal

[epixa]

I think we need to add a build step to convert the sass to css so we're not shipping libsass and compiling sass via webpack in production builds.