Revert eleveldb to nhs-develop-3.0

[martinsumner]

The nhs-develop branch uses the updated nhs-develop eleveldb (currently), which contains a number of changes, in particular to the installation of snappy.

For the next nhs release the preference is instead to continue to use the pre-defined snappy in-built with eleveldb, and avoid changes with the build process - using the nhs-develop-3.0 branch of eleveldb instead. The nhs-develop-3.0 branch can be made compatible with OTP 26 OpenRiak/eleveldb#2 - so there is no real need to change (especially as it is likely eleveldb will be removed before the next major release, or perhaps switched to the branch of another Riak user).

This will also require a change in nhs-riak/riak_ensemble.

[martinsumner]

https://github.com/nhs-riak/riak_core/pull/9/files