Symfony3 compatibility

[hacfi]

• allow Symfony ~3.0
• bump Symfony dependency to ~2.7
• specify correct Sonata dependencies
• use parentDocument in Admin classes
• update route_basepaths in tests
• make StyleCI happy

See #331

[wouterj]

May I do one last try? :)

DEPS=dev SYMFONY_VERSION="3.0.x-dev as 2.8.x-dev"

[dbu]

or i just merge and we can do further PRs

[dbu]

thanks a lot @hacfi
i merged this so that we can do independent PRs to work on the build matrix and getting rid of deprecated / BC things.

[wouterj]

We now have a symfony3 and symfony_3 branch, I think the one with the underscore has to be removed?

[dbu]

ooops. sorry, did not notice that one (in my defense, there is no open PR for it, either). most of symfony_3 is about adding BC for the forms, which is not needed now that we never support 2.3 and 3.0 with the same version. so i guess we can simply delete that. however, the change 00c9db4 might be relevant, or not? it actually should go into the 1.4 version, it is not about the symfony version.