updated release date for Swarm beta

[Nelson Minar]

Because significant amounts of new code are still being added to the
alpha version of Swarm, we've decided to delay the general beta release
from June 1 until later in the summer.

During the alpha phase, we decided to improve Swarm's model of
scheduling and concurrency, and further develop the the interface for
getting data into and out of experiments.  These changes are important,
but require a delay in release.  We will continue testing of Swarm on
selected alpha projects through the summer and aim at a public beta
release as soon as possible.