Your requirements could not be resolved to an installable set of packages. Problem 1 - guzzlehttp/guzzle[6.5.8, ..., 6.5.x-dev] require guzzlehttp/psr7 ^1.9 -> found guzzlehttp/psr7[1.9.0, 1.9.1, 1.9.x-dev] but it conflicts with your root composer.json require (^2.0). - guzzlehttp/guzzle[6.0.0, ..., 6.0.1] require guzzlehttp/psr7 ^1.0.0 -> found guzzlehttp/psr7[1.0.0, ..., 1.9.x-dev] but it conflicts with your root composer.json require (^2.0). - guzzlehttp/guzzle[6.0.2, ..., 6.2.0] require guzzlehttp/psr7 ~1.1 -> found guzzlehttp/psr7[1.1.0, ..., 1.9.x-dev] but it conflicts with your root composer.json require (^2.0). - guzzlehttp/guzzle[6.2.1, ..., 6.2.2] require guzzlehttp/psr7 ^1.3.1 -> found guzzlehttp/psr7[1.3.1, ..., 1.9.x-dev] but it conflicts with your root composer.json require (^2.0). - guzzlehttp/guzzle[6.2.3, ..., 6.3.3] require guzzlehttp/psr7 ^1.4 -> found guzzlehttp/psr7[1.4.0, ..., 1.9.x-dev] but it conflicts with your root composer.json require (^2.0). - guzzlehttp/guzzle[6.4.0, ..., 6.5.7] require guzzlehttp/psr7 ^1.6.1 -> found guzzlehttp/psr7[1.6.1, ..., 1.9.x-dev] but it conflicts with your root composer.json require (^2.0). - php-http/guzzle6-adapter 2.x-dev is an alias of php-http/guzzle6-adapter dev-master and thus requires it to be installed too. - php-http/guzzle6-adapter dev-master requires guzzlehttp/guzzle ^6.0 -> satisfiable by guzzlehttp/guzzle[6.0.0, ..., 6.5.x-dev]. - Root composer.json requires php-http/guzzle6-adapter 2.x-dev -> satisfiable by php-http/guzzle6-adapter[2.x-dev (alias of dev-master)].