Downgrade commons-io dependency so that it's compatible with sbt-idea

Fixes #2882.

When sbt-idea is a global plugin it brings in an old version of
commons-io. This change downgrades Play's version to match sbt-idea's
version so that Play's sbt-plugin and sbt-idea can run in the same classloader
as each other.
Merge pull request #2891 from richdougherty/commons-io-version

Downgrade commons-io dependency so that it's compatible with sbt-idea