Fixed bug where all remote extensions were displayed as upgradable.

Created by: universalhandle

This change does the trick.

It does result in following edge case, where I have an old version of CiviDiscount downloaded and a newer version is available. I think displaying the option to upgrade a disabled/uninstalled extension is a choice, not a bug. Argument for doing it this way:

Joe was very excited to try CiviDiscount 3.4, but he found a bug which presented so much of a problem for him that he decided to disable it. Oh, hey, 3.7 is out -- maybe his bug report has been addressed!