Increase default memory size to 2G

This avoids running out of /tmp space during the integration tests
which run as part of Docker's `make test`.

See https://github.com/dotcloud/docker/issues/6331.
Update docs for 2G memory default

Docker-DCO-1.1-Signed-off-by: Glyn Normington <gnormington@gopivotal.com> (github: glyn)
Merge pull request #151 from glyn/master

Increase default memory size to 2G