Dynamically loading jQuery when it's not available or version isn't high enough
Before actually diving into creating a jQuery checker (nice idea by the way), you might really want to check some cases before building the code. There are several issues with your script here:


Your script assumes that there is jQuery. What if there's no jQuery at all? How would your script work? I suggest you stick to vanilla JS instead.
Documentation too long, didn't read, developer loaded your script first. It will break. You can't detect scripts because they are parsed in order. If your script came first, it cannot see the scripts loaded after it at the point of execution.
But what if a plugin already initialized with the version that came with the page? jQuery carries an internal cache for data, event handlers etc. If you switch jQuery now, they will break.
Dynamically loading a script is an async operation. You can't pause it right there. JS will continue. Therefore, while your script is loading a more recent version, plugins on your page might have already initialized using the one in your page.
Some plugins require a specific version of jQuery (like some prefer 1.4, or 1.6). Developers intentionally use them because some of these plugins use already deprecated API and weren't updated. If you load a newer version, they break.
If you really wanted IE support, why not just load the 1.x version rather than doing 2.x? This removes the need to detect IE. That's why there's a 1.x and 2.x version in the first place.