DRILL-4285: Bumps FMPP version to 0.9.15/FreeMarker to 2.3.21
FMPP 0.9.14 pom.xml specifies a version range for FreeMarker dependency,
which should be okay except that latest SNAPSHOT version doesn't resolve
anymore.
FMPP 0.9.15 now uses a fixed stable version (2.3.21).
This closes #330.
