Drop support for HBase 0.94

HBase 0.94 contains many implementation differences vs. 0.96+, which make implementing new features much more difficult. Since HBase 0.94 does not seem to be actively used with Tephra, we should drop support for it and focus only on supporting newer versions.

Link to mailing list discussion around dropping HBase 0.94 support: https://groups.google.com/forum/#!topic/tephra-dev/yMp60ti2lPA