Revert "app-emulation/docker: bump to v1.3.1"

This reverts commit 87efde004420ce93bca42d531d140d05150e44ba.
Bumps docker-1.3.1.ebuild -> docker-1.3.0-r1.ebuild.
Merge pull request #954 from crawford/docker

Revert "app-emulation/docker: bump to v1.3.1"