Updated requirements in composer.json to "~3.1" to support 3.1.0 stable.

See https://groups.google.com/forum/#!topic/silverstripe-dev/kCHSHLZp2Yg
Merge pull request #49 from purplespider/patch-1

Updated requirements in composer.json to "~3.1" to support 3.1.0 stable.