Changes:
- Updated to Guzzle5 (#79: @Nyholm)
- Updated default API version from v2 to v3 (#75: @Ezo)
- Show response message on 400, 401 and 404. (#72: @CupOfTea696)
- PHP DocBlocks, Constants Changes, and Minor Refactors (#66: @cerealean)
- Added PHP 7.0 support for Travis-CI, removed PHP 5.3 support (#79: @Nyholm)
Thanks for the contributions, everybody!