kiji-mapreduce-lib: Exclude unnecessary jars from packaging.

Use commons-codec version used by other Kiji components.
Exclude distribution-specific kiji-avro from lib dir.
commit: https://github.com/kijiproject/kiji-mapreduce-lib/commit/a7e935ea78d368ca5cb000ad0993754b6cfcb869
kiji-mapreduce: Update commons-codec to 1.6 (to match other Kiji components).

commit: https://github.com/kijiproject/kiji-mapreduce/commit/c818011365c03c97721332fc85b0423c7913242a
kiji-poms: Add commons-codec:commons-codec:1.6 to dependency mgmt.

commit: https://github.com/kijiproject/kiji-poms/commit/ec807a7b554ef70a4b64385603b2631f00c2ffec