Do not require hard versions

Requiring a specific exact version is a really bad idea. If Guzzle release 3.6.1 which contains an important security update, then anyone using your version is completely stuck until they:

1. You update to 3.6.1
2. They delete your package

Guzzle is on 3.8 already, so upgrading to require `3.8.*` would be really helpful.
Merge pull request #1 from philsturgeon/patch-1

Do not require hard versions