[SPARK-17378][BUILD] Upgrade snappy-java to 1.1.2.6

## What changes were proposed in this pull request?

Upgrades the Snappy version to 1.1.2.6 from 1.1.2.4, release notes: https://github.com/xerial/snappy-java/blob/master/Milestone.md mention "Fix a bug in SnappyInputStream when reading compressed data that happened to have the same first byte with the stream magic header (#142)"

## How was this patch tested?
Existing unit tests using the latest IBM Java 8 on Intel, Power and Z architectures (little and big-endian)

Author: Adam Roberts <aroberts@uk.ibm.com>

Closes #14958 from a-roberts/master.