[Tests/*] Added some additional test placeholders
[composer.json] Allow only stable requires
[composer.json] Loosen requirement versions

guzzle/http: >=3.4,<4.0 instead of >=3.4.0,<3.5.0
phpunit/phpunit: >=3.7,<4.0 instead of >=3.7.0,<3.8.0