This PR was squashed before being merged into the 2.1.x-dev branch (closes #1523).

Discussion
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Test all the 3.x branches in Travis

I enhanced the Travis config to ensure the tests are run against all the available 3.x branches.

Before the changes:
- The tests were run against `2.8` when `$SYMFONY_DEPS_VERSION` was unset
- There were additional envs for `3.0`, `3.1` and `3.2`

After the changes:
- When `$SYMFONY_DEPS_VERSION` is unset, `composer.json` is left unchanged and the tests are run against the most recent `3.x-dev` branch (what you'd get by running phpunit locally)
- There are envs to test all the symfony branches from `2.8` to `3.3`

I also fixed the cache (there were two `cache/directories` sections) and made the syntax more consistent overall.

This changes the behaviors of the build: before, `Twig 2.x` was tested with `2.8`. Now, it is tested alongside the most recent `3.x`. I believe it makes more sense like this, however if the old behavior was on purpose (i.e because `2.8` is still the most recent _lts_), I can still switch back and add envs for `3.3` and `3.4` instead.

Regarding the changes to the tests:

I originally removed `symfony/web-link` from the list of dependencies when testing against `< 3.3`, as something like this will be necessary later when adding `symfony/cache` support which conflicts with `symfony/var-dumper < 3.3`.

However, as this was not necessary and actually prevented us to check that the WebLink integration didn't break when the component was available but associated with an old version of the `twig-bridge`, I ultimately rolled back the change. Yet, I kept the checks in the tests as they are cleaner this way IMO (anyway they're in a separate commit so I could just remove them too if you want).

Commits
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5557944d Test all the 3.x branches in Travis