Suggest guzzle/guzzle instead of require it

[iamluc]

The hard dependency on Guzzle 3 blocks the usage of newer versions.

I would like to suggest the package instead of require it as an hard dependency.

[Nek-]

I agree this is a problem. A more serious fix is to switch to guzzle 6.

Also this fix is invalid because there is nothing in the code that test that guzzle is actually here in the good version. Usage of FriendlyContext will result in weird errors.

A last thing about this patch is that people using Symfony3 will never be able to use he API part (which is probably the best part of friendlycontext). This will generate frustration, issues, and so on.

[iamluc]

I used the EntityContext succesfully using my fork, so the extension is loaded correctly by behat even if guzzle is not installed.
But I agree we could need more checks to avoid some crashes.

The ApiContext will still be usable, but users will have to require guzzle themself. That's why I modified the documentation: https://github.com/KnpLabs/FriendlyContexts/pull/223/files#diff-b89a5bb9165e004541d763d27d5c7da4

For Guzzle 6, I started the migration (#224) but I don't have the time right now (And I don't use the ApiContext).

Finally, to avoid similar issues (What I do if I use guzzle 5 on my project ?) in the future, I think it is a good idea to make some dependencies optionals.

[Nek-]

I agree with this PR. Agree with the fact that a fork is needed. Agree with guzzle 6 update. Agree with dropping dependencies...

I just think dependencies missing errors really need to be handle. :-)