We currently officially support Jackson 2.4, but some users wish to
use Jackson 2.5. This causes a failure as JacksonAutoConfiguration
depends on JacksonJodaFormat which exists in 2.4 but not in 2.5. This
commit updates JodaDataTimeJacksonConfiguration to make it conditional
on JacksonJodaFormat being on the classpath. This means that Jackson
2.5 users will not have configurable DateTime formatting
(added in 201fb5e5) but things will generally work once again.

Closes gh-2573