adjusted composer.json to follow more stable versioning policy for assets

as discussed before, better do this change before RC to avoid breaking BC in GA.

composer.json

@@ -19,7 +19,7 @@
     ],
     "require": {
         "yiisoft/yii2": "*",
-        "bower-asset/bootstrap": "3.2.* | ~3.1"
+        "bower-asset/bootstrap": "3.2.* | 3.1.*"
     },
     "autoload": {
         "psr-4": {