Support common client 2.x

[Nyholm]

Follow up on #291

This uses the forward compatibility layer introduced in php-http/client-common#132

[dbu]

this will need a release of client-common. looks good to me, lets merge once we tagged 1.9 of client-common.

[Nyholm]

I've tested this and it is working.

I also added

     "prefer-stable": true,
     "minimum-stability": "dev"

Which means that we dont need to wait for a release of 1.9 of client-common to actually merge this.

[Nyholm]

We allow too many versions. That is why Travis is failing: https://travis-ci.org/php-http/HttplugBundle/jobs/473334912

I updated all symfony related versions. We only support the latest patch version for all supported minor versions.

[sowbiba]

I cannot install latest version of httplug-bundle because

  Problem 1
    - Installation request for php-http/httplug-bundle 1.14.0 -> satisfiable by php-http/httplug-bundle[1.14.0].
    - php-http/httplug-bundle 1.14.0 requires php-http/cache-plugin ^1.6 -> satisfiable by php-http/cache-plugin[1.6.x-dev] but these conflict with your requirements or minimum-stability.

client-common 1.9 is released, will cache-plugin be released soon ? 🙏

Thanks

[Nyholm]

Yeah, we are waiting for other releases. You could try it out by adding the following in composer.json:

"php-http/cache-plugin": "dev-master as 1.7",
"php-http/logger-plugin": "dev-master as 1.7",
"php-http/stopwatch-plugin": "dev-master as 1.7"

They should all be released as soon as possible.
If you want to help, you can review these 3 plugins and update the change logs.