NOTE: don't merge it before we cut stable jQuery 3.0.0!

This PR is a preparation for the browser-support site changes that are needed for jQuery 3.0.0.

Apart from updating browsers' versions, I made a couple of changes:

1. I grouped Chrome, Firefox & Opera into one column; with the addition of Edge the table got really cramped and it may still get more entries if we decide to officially test on a Windows Phone.
2. I moved jQuery to be first & jQuery Compat to be second. At this point where we're starting discussing dropping IE8 in some not-so-absurdly-distant future, with additional platform support like Node (it wasn't really properly tested in 2.x) and, last but not least, considering that the name jQuery hints it is the main line it made sense to me to change the "default" version to choose.
3. Some people (hopefully not a lot) will still need to support mammoths like IE6 so I added a paragraph directing to the last version that supports them.
4. I changed (Current - 1) or Current to (Current - 1) and Current as we support both versions, not one of the other.

Edge is presented separately as we don't know if BrowserStack will make more than one build available. Edge will not have official version numbers but its user agent contains the build number in the form of Edge/12.BUILD_NUMBER; current build is 9600. If more than one build will be made available, I'd merge this column with the Chrome one.

Screenshots:

1. current page:
2. page with this PR applied:

cc @jquery/core

Things to consider:

• remove the "CSS contents and browser compatibility" link from "External References" at the bottom of the page; it was mostly relevant for IE<8 and most of the table has since been moved to another place.