minor #41228 Introduce SYMFONY_FEATURE_BRANCH variable in Travis config (derrabus)
minor #41228 Introduce SYMFONY_FEATURE_BRANCH variable in Travis config (derrabus)

This PR was squashed before being merged into the 4.4 branch.

Discussion
----------

Introduce SYMFONY_FEATURE_BRANCH variable in Travis config

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | #40887
| License       | MIT
| Doc PR        | not needed

This PR proposes to introduce a `SYMFONY_FEATURE_BRANCH` variable that always points to the branch of Symfony's next feature release. This is a setting that we need to adjust twice a year. This way, the branch does not have to follow the `.x` suffix convention anymore.

~~I'm currently testing my changes against the 5.x branch. As soon as the PR is ready, I will target 4.4.~~

Update: `SYMFONY_FEATURE_BRANCH` is now populated via an API request.

Commits
-------

7de11c0e44 Introduce SYMFONY_FEATURE_BRANCH variable in Travis config

symfony/symfony,nicolas-grekas/symfony,damienalexandre/symfony,symfony/symfony,phramz/symfony,tucksaun/symfony,phramz/symfony,Slamdunk/symfony,curry684/symfony,ogizanagi/symfony,xabbuh/symfony,hhamon/symfony,derrabus/symfony,tucksaun/symfony,zerkms/symfony,damienalexandre/symfony,Tobion/symfony,derrabus/symfony,xabbuh/symfony,ogizanagi/symfony,Tobion/symfony,ro0NL/symfony,paradajozsef/symfony,Tobion/symfony,ro0NL/symfony,mpdude/symfony,HeahDude/symfony,hhamon/symfony,ogizanagi/symfony,derrabus/symfony,d-ph/symfony,stof/symfony,sgehrig/symfony,stof/symfony,nicolas-grekas/symfony,mpdude/symfony,oleg-andreyev/symfony,sgehrig/symfony,ro0NL/symfony,d-ph/symfony,damienalexandre/symfony,nicolas-grekas/symfony,tucksaun/symfony,curry684/symfony,d-ph/symfony,zerkms/symfony,damienalexandre/symfony,ogizanagi/symfony,paradajozsef/symfony,nicolas-grekas/symfony,Taluu/symfony,paradajozsef/symfony,sgehrig/symfony,mpdude/symfony,HeahDude/symfony,tucksaun/symfony,Tobion/symfony,Slamdunk/symfony,Slamdunk/symfony,ro0NL/symfony,phramz/symfony,curry684/symfony,xabbuh/symfony,curry684/symfony,derrabus/symfony,paradajozsef/symfony,Taluu/symfony,symfony/symfony,Taluu/symfony,oleg-andreyev/symfony,sgehrig/symfony,oleg-andreyev/symfony,HeahDude/symfony,xabbuh/symfony,oleg-andreyev/symfony,symfony/symfony,stof/symfony,mpdude/symfony,d-ph/symfony,stof/symfony,zerkms/symfony,Taluu/symfony,phramz/symfony,hhamon/symfony,Slamdunk/symfony,HeahDude/symfony,hhamon/symfony,zerkms/symfony