Jackson-jr 2.10 accidentally uses `UncheckedIOException` only available on JDK 8

[cowtowncoder]

Looks like #60 implementation added unfortunate JDK 8 dependency via use of UncheckedIOException which while otherwise nice and useful (esp. for this use) can not be used if we are to retain Java 6 compatibility.
So need to remove from 2.10.2, re-add in 3.0.