Problems with Guzzle 6.

I'm using Guzzle 6 and getting these errors while trying to install:

Your requirements could not be resolved to an installable set of packages.

Problem 1
- pragmarx/zipcode v1.5.2 requires guzzlehttp/guzzle ~4|~5 -> satisfiable by guzzlehttp/guzzle[4.0.0, 4.0.0-rc.1, 4.0.0-rc.2, 4.0.1, 4.0.2, 4.1.0, 4.1.1, 4.1.2, 4.1.3, 4.1.4, 4.1.5, 4.1.6, 4.1.7, 4.1.8, 4.2.0, 4.2.1, 4.2.2, 4.2.3, 5.0.0, 5.0.1, 5.0.2, 5.0.3, 5.1.0, 5.2.0, 5.3.0, 5.3.x-dev] but these conflict with your requirements or minimum-stability.
- pragmarx/zipcode v1.5.1 requires guzzlehttp/guzzle ~4.0 -> satisfiable by guzzlehttp/guzzle[4.0.0, 4.0.0-rc.1, 4.0.0-rc.2, 4.0.1, 4.0.2, 4.1.0, 4.1.1, 4.1.2, 4.1.3, 4.1.4, 4.1.5, 4.1.6, 4.1.7, 4.1.8, 4.2.0, 4.2.1, 4.2.2, 4.2.3] but these conflict with your requirements or minimum-stability.
- pragmarx/zipcode v1.5.0 requires guzzlehttp/guzzle ~4.0 -> satisfiable by guzzlehttp/guzzle[4.0.0, 4.0.0-rc.1, 4.0.0-rc.2, 4.0.1, 4.0.2, 4.1.0, 4.1.1, 4.1.2, 4.1.3, 4.1.4, 4.1.5, 4.1.6, 4.1.7, 4.1.8, 4.2.0, 4.2.1, 4.2.2, 4.2.3] but these conflict with your requirements or minimum-stability.
- Installation request for pragmarx/zipcode ^1.5 -> satisfiable by pragmarx/zipcode[v1.5.0, v1.5.1, v1.5.2].

Installation failed, reverting ./composer.json to its original content.