Bitly4laravel Versions Save

No changes in the code, the README file with installation instructions has been modified.

Updated the README file, refs #27, #34.

Resolved #35

Change log:

• changed minimum required php version in composer to 5.6
• merged #31 with slight modifications
• removed from travis php 5.4, 5.5

Reverted composer minimum stability to dev and added prefer-stable option #27.

Resolved: #27 Improved: #32 Added php 7.1, 7.0, 5.6 to Travis.ci config

Resolves #32

• support for Guzzle 6 added

In this update an issue was fixed when the params provided by make() method did not override the default params set in the configuration. Thanks to shivergard this issue has been identified and fixed.

• fixing #16