Update riak to 1.3.1

Changes in 1.3.1:
- 2i Big Integer Encoding
- Improved bitcask startup time
- Fix behaviour of PR/PW
- Bugfixes

Changes in 1.3.0:
- Active Anti-Entropy
- Additional IPv6 support
- Luke Removal
- 'riak getpid' Added
- Riaknostic Included by Default
- Health Check
- Reset Bucket Properties
- Support for logging to syslog
- Bugfixes

Full changelog:

https://github.com/basho/riak/blob/master/RELEASE-NOTES.md