Use simple-phpunit to run tests on travis-ci

[fbourigault]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	partially #175
Documentation	N/A
License	MIT

What's in this PR?

This remove the phpunit/phpunit dev dependency and use symfony/phpunit-bridge simple-phpunit wrapper to run tests. PHPUnit installation is also cached.

I also moved global environment variables under the env.global .travis.yml section. Those variables are now hidden from the jobs list. See latest master build and current build for comparaison.

I also bumped matthiasnoback/symfony-dependency-injection-test minimum version to ^1.0.1 to avoid having phpunit/phpunit installed as dependency which caused some autoloading mess.

It looks like this was forgotten when doing #149.

[Nyholm]

Thank you.
Tell me why simple-phpunit is better? What is the win of this PR?

[fbourigault]

My bad. I get confused myself by depending on symfony/phpunit-bridge for deprecation helper and using simple-phpunit.

There is no added value for php-http/HttplugBundle at the moment.

[Nyholm]

Don't close it just because I do not understand. :/

[fbourigault]

I didn't closed because you didn't understand. I thought there was an issue by using vendor/bin/phpunit directly but there is none. So using vendor/bin/simple-phpunit over vendor/bin/phpunit has 0 added value here.