Increment project version to 0.1.5

Changes since the last version:

- Re-implemented the `npmap` function in `io.mandoline.utils` using the
  Claypoole library

- Fixed a race condition in `CachingChunkStore`

- Added Leon Barrett to the list of contributors in README
Fix concurrency test

The concurrency test was using Mandoline's internal threadpool and
causing contention for the threads, making the test block. This change
fixes that.
Merge pull request #9 from leon-barrett/concurrency-fix

Fix concurrency test
Bump version to 0.1.6

- Change utils/npmap to claypoole's pmap in
io.mandoline.backend.dynamodb-test/ddb-lots-of-overlaps
Merge pull request #10 from TheClimateCorporation/release-0.1.6

Bump version to 0.1.6