Update requirements to depend on requests>=1.1.0

The latest version of requests is 1.2.3 and pinning requests-kerberos's
required version to exactly 1.1.0 means it can't be used by an app that
already has a pinned requirement on requests 1.2.3

Refs #14
Merge pull request #18 from dpursehouse/update-requirements

Update requirements to depend on requests>=1.1.0