Fix no required Piwik version was defined (#10413)

* Fix no required Piwik version was defined

* plugins should specify an upper version constraint to the next major version

plugins/ExamplePlugin/plugin.json

@@ -4,7 +4,7 @@
   "description": "Piwik Platform showcase: how to create widgets, menus, scheduled tasks, a custom archiver, plugin tests, and a AngularJS component.",
   "theme": false,
   "require": {
-    "piwik": ">=PIWIK_VERSION"
+    "piwik": ">=PIWIK_VERSION,<3.0.0-b1"
   },
   "authors": [
     {

plugins/ExampleTheme/plugin.json

@@ -3,6 +3,9 @@
     "description": "Piwik Platform showcase: example of how to create a simple Theme.",
     "version": "0.1.0",
     "theme": true,
+    "require": {
+       "piwik": ">=PIWIK_VERSION,<3.0.0-b1"
+    },
     "stylesheet": "stylesheets/theme.less",
     "homepage": "",
     "license": "GPL v3+",