Change 'play' command to bootstrap SBT with Scala 2.10.3

If a bootstrap setting isn't supplied then SBT will usually default to
Scala 2.10.2. That messes up our all-in-one ZIP distribution which we
want to ship with Scala 2.10.3 only.

The 'build' script needs to support bootstrapping SBT with versions
other than 2.10.3 (e.g. when building Play itself), therefore it's
default Scala version is 'auto'. It will only use a version of 2.10.3
when called from the 'play' script. The 'play' script sets an
environment variable to override 'build's default value.