guzzle http psr7 2.0 compliant

[riccafi]

PR checklist

• [x ] Read the contribution guidelines.
• [x ] Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• [x ] Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• [x ] Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

@dkarlovi (2017/07) @mandrean (2017/08)

Description of the PR

Fixed file api.mustache for PHP language so that it's used Query::build class method instead of build_query function .
Updated samples that were using this build_query function.

[rajatsib]

Hi @HugoMario

Can you release a new tag for the changes merged in this PR ?