Fix Scala 2.10 compilation warnings

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
fix: Relax DateTime tag type constraint

There is the MonthYearField in addition to the UtcTimestampField.

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Merge pull request #91 from jvirtanen/features/date-time-tag-type-constraint

Relax DateTime tag type constraint
Merge pull request #92 from jvirtanen/fixes/scala-2-10-compilation-warnings

Fix Scala 2.10 compilation warnings