Use the new build env on Travis

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Fix for Symfony 2.6
Documentation fix
Merge pull request #202 from mvar/doc_fix

Documentation fix
[command] add `payum:payment:debug` command.
fix tests on symfony 2.6
[command] add docs about payument debug command.
[debug] show supports method code declaration.
[debug] add --show-supports options to payment debug cmd.
Merge pull request #201 from mvar/symfony_2_6

[bugfix] Symfony 2.6 support
Merge pull request #200 from joshk/patch-1

Use the new build env on Travis
Merge pull request #204 from formapro-forks/payment-debug-cmd

Debug payment command.
Release 0.12.3
Merge branch '0.12'

Conflicts:
	composer.json
[payment-builder] rework paypal express checkout factory.
[payment-builder] use payment factory method to create builder, and builder to create payment.
wip