You might have installed the Lightroom CC 2015 upgrade, but I believe you still have your older version installed. If you check Help->About Lightroom, I think you will find that you are still running an older version. The icons for the two programs are the same. Installing a new upgrade of Lightroom does not replace the older version. So it's up to you to get things sorted out.
A number of the photographers that I assist in private one-on-one Adobe Photoshop Lightroom technical support sessions have been reporting a similar error message lately. Each of these clients reports receiving an error message that says that their Adobe Photoshop Lightroom Catalog "cannot be opened because it is too new to be used by this version of Lightroom." Usually this error message appears a few day after installing a Photoshop Lightroom update or upgrading from an older version of the Lightroom application.
Fortunately, there is a quick and easy solution to this problem. If you encounter this error message on startup then all that you need to do is to tell your computer to open this Lightroom Catalog (your .lrcat file) using the most recently installed version of the Lightroom Application.
In the Mac world, this is best done by right-clicking on your .lrcat file and using the <em>Open With</em> command. Windows users will find the same command with a right-click although so far I have mostly seen this error on computer's running the Mac operating system. See the video tutorial below for more assistance resolving this issue.