Update dependencies to supported version and update travis config

[acrobat]

• removed a useless test as we are testing symfony behavior and this is already covered in tests of the symfony framework itself

[sagikazarmark]

Any reason for removing the multiple symfony version test?

[acrobat]

I don't think it's necessary to test each symfony version because of their BC policy. So testing with the composer flag --prefer-lowest will give us symfony 2.7.* components and with a normal update it will give us the highest stable component versions. So this way we got everything covered with minimal tests. If I don't overlook anything of course :)

[sagikazarmark]

AFAIK that BC promise has been broken a few times (think about the form component changes around 2.4-2.6)

I checked other libraries and there seem to be a trend to remove these kind of tests, so you might be right after all.

For reference, here is HTTPlug Bundle's travis config, which is kind of a midway: https://github.com/php-http/HttplugBundle/blob/master/.travis.yml

[acrobat]

Ah yes I wanted to check on that bundle! I will update the PR to added some extra tests like in the HTTPlugBundle!

[acrobat]

@sagikazarmark I've tweaked the travis config by reference of the HTTPlugBundle config