Changed version constratints to be semantic versioning again

- in composer the `~` operator is just a shortcut for `~1.0` = `>=1.0.0,<2.0.0`
  it does not prevent dev versions to be installed but neither does `1.*`
- dev versions are constrolled via `minimum-stability` setting and the
  `@dev`, `@beta`, `@stable`,... settings.
- setting bower packages that ship compiled files only with the stable
  release to use `@stable` in composer.json of the applictaions. This
  setting can be removed when we remove the `minimum-stability=dev`
  setting after GA.

composer.json

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