bower.json: main: select2.css => core.scss

Per bower/bower.json-spec#43 :
> Use source files with module exports and imports over pre-built distribution files.

Also, the example in that PR includes
/sass/motion.scss in and excludes
/dist/movement.css & /dist/movement.min.css
from its `main`.

bower.json

@@ -3,7 +3,7 @@
     "description": "Select2 is a jQuery based replacement for select boxes. It supports searching, remote data sets, and infinite scrolling of results.",
     "main": [
         "dist/js/select2.js",
-        "dist/css/select2.css"
+        "src/scss/core.scss"
     ],
     "repository": {
         "type": "git",