bower.json: Remove bootstrap.css from `main`

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

Also, the example in that PR doesn't include
/dist/movement.css or /dist/movement.min.css
in its `main`.

If you have a complaint about this, take it up with the Bower folks:
https://github.com/bower/bower.json-spec/issues

bower.json

@@ -15,7 +15,6 @@
   "homepage": "http://getbootstrap.com",
   "main": [
     "less/bootstrap.less",
-    "dist/css/bootstrap.css",
     "dist/js/bootstrap.js"
   ],
   "ignore": [