Maintenance of different symfony versions

[aivus]

Hi guys.

It's hard to support few major versions of SF in one branch (requires some hacks with Kernel::VERSION and with testing).
See Extending to Test Three Versions in https://pantheon.io/blog/highest-lowest-testing-multiple-symfony-versions

It's better to use different branches for different major versions.

What do you think about creating separate branches 2.0 (with support of sf 2.x), 3.0 (with support of sf 3.x) and so on?

/cc @Spike31