Upgrade Snappy to 1.1.6

We currently use snappy v1.1.1. A number of performance improvements have been made to compression & decompression speed in subsequent releases, in particular in v1.1.4 - from https://github.com/google/snappy/blob/master/NEWS:

Snappy v1.1.4, January 25th 2017:

• Fix a 1% performance regression when snappy is used in PIE executables.

• Improve compression performance by 5%.

• Improve decompression performance by 20%.

Given that snappy performance plays a major part in couchstore read/write performance; upgrade to the most recent release (v1.1.6) to take advantage of these speedups.