Unable to install using composer

[coreorm]

Hi Dave,

Below are the details of the error:

Your requirements could not be resolved to an installable set of packages.
  Problem 1
    - The requested package davec49/silex2-simpleuser ^2.0 exists as davec49/silex2-simpleuser[dev-master, dev-silex2.1] but these are rejected by your constraint.

and the composer.json is like:

"davec49/silex2-simpleuser": "^2.0",

Please help, thanks.