Add missing opaque module dependency

Fixes:

    voxel-texture $ node index.js
    
    module.js:340
        throw err;
              ^
    Error: Cannot find module 'opaque'
        at Function.Module._resolveFilename (module.js:338:15)
        at Function.Module._load (module.js:280:25)
        at Module.require (module.js:364:17)
        at require (module.js:380:17)
        at Object.<anonymous> (/private/tmp/node_modules/voxel-texture/index.js:3:21)
        at Module._compile (module.js:456:26)
        at Object.Module._extensions..js (module.js:474:10)
        at Module.load (module.js:356:32)
        at Function.Module._load (module.js:312:12)
        at Function.Module.runMain (module.js:497:10)