Add symfony 3.0 support

[GuilhemN]

This PR adds symfony 3.0 support.

[GuilhemN]

Must be removed once kriswallsmith/assetic#772 is merged.

[stof]

it is merged now

[GuilhemN]

The build failure is unrelated to this PR.

[stof]

please don't remove the test against dev dependencies

[GuilhemN]

Should we allow this test to fail ?

[GuilhemN]

Do you want to wait until https://github.com/kriswallsmith/assetic is released?

[mmoreram]

This PR doesn't assure the compatibility with Symfony3 until you add the dependency with kriswallsmith/assetic to ^1.3.2 (not released yet)

[GuilhemN]

There is no need to use this constraint as composer will manage this automatically.

[mmoreram]

Your composer file must manage its own dependencies. If you install this component using prefer-lowest, then version old assetic version will be installed, so it will not cover this feature.

ping @stof

[GuilhemN]

If an user wants to use symfony 3.0, composer will only install the versions of the dependencies that are compatibles. If you use prefer-lowest, composer will install kriswallsmith/assetic 1.3.2 because this is the oldest version compatible.
If the user wants to use symfony 3.0 with kriswallsmith/assetic <1.3.2 composer will return an error.

[mmoreram]

Not agree.
This bundle cannot have external visibility of it's dependencies dependencies.
Accepting this PR you are saying (imagine that this PR is merged before 2.7.1)

• Hey guys! If you install Symfony 3.0.0, you can use our version AsseticBundle 2.7.1.
• Your bundle MUST ensure that this is right, no matter what happens outside.
• This bundle allows kriswallsmith/assetic to be in version ^1.3.1, which has a deprecation element (Globals without interface)... and this will be not valid anymore in Symfony 3.0.0.

So, what you have to do here is increment your dependency with assetic library, and if other users need Symfony 2.x.x then composer will install maximum AsseticBundle 2.7.0

[GuilhemN]

kriswallsmith/assetic wasn't compatible with sf3.0 before kriswallsmith/assetic#772 which resolves all the deprecations.

An example:

{
    "require": {
        "php":                            ">=5.3.9",
        "symfony/symfony":                "^3.0@dev",
        "symfony/assetic-bundle":         "^2.0",
        "kriswallsmith/assetic":          "^1.3"
    }
}

With this composer.json, if you run composer update --prefer-lowest, composer will install:

• symfony/symfony 3.0.x-dev
• symfony/assetic-bundle 2.7.1 (I suppose that this PR will be merged before 2.7.1)
• kriswallsmith/assetic 1.3.2

Because if for example composer installs symfony/assetic-bundle 2.7.0, there would be a conflict between the composer.json of symfony/assetic-bundle and the version of symfony/symfony.
If composer installs kriswallsmith/assetic 1.3.1, there would be the same issue: a conflict between the composer.json of kriswallsmith/assetic and the version of symfony/symfony.

[stof]

@mmoreram don't confuse Twig 2.x and Symfony 3. Symfony 3 still allows Twig 2

[stof]

The remaining test failure on PHP 7 is because of sebastianbergmann/phpunit-mock-objects#267

[stof]

Thank you @Ener-Getick.