title: Upgrade Sonar when plugins require to be upgraded
body: <p>I'm trying to upgrade from Sonar 4.1.2 to 5.1.1 (but the problem would be the same if I wanted to upgrade to 4.2).
  I use some plugins that needs to be upgraded. For example Android plugin should be upgraded from 0.1 to 1.1 (or 1.0 to upgrade to Sonar 4.2).</p>
  
  <p>The sonar System updates page (my_sonar_server_url/updatecenter/system_updates) tells me to first upgrade the plugin, and shows me an "upgrade" button in front of that plugin. But clicking on it results in a message "Plugin android is needed to be installed at version greater or equal 1.1".<img src="https://i.stack.imgur.com/GDgTt.png" alt="enter image description here"></p>
  
  <p>I tried to go in the "Plugin Updates" tab, where the plugin upgrades are marked "Incompatible, requires to upgrade system".<img src="https://i.stack.imgur.com/jOtoZ.png" alt="enter image description here"></p>
  
  <p>This seems inconsistent, and I wonder how I can make the upgrade.
  Looking at <a href="http://docs.sonarqube.org/display/SONAR/Upgrading" rel="nofollow noreferrer">http://http://docs.sonarqube.org/display/SONAR/Upgrading</a> I presume I have to "manually upgrade the plugins". But how I can do that? Where can I download the required plugin version?</p>