Update tools.mk to v0.5.5

This also disregards the bitcask-specific tools.mk change made in
755cf482689ce887df35960cfcc2c6fdd70209b0. Per @seancribbs advice, we
simply add a prerequisite to the `test` target, in the form of
`eunit_nif`. The `erlang` version runs because we globally set
BITCASK_IO_MODE=erlang. Testing this with `make -n test`, we see:

    ./rebar compile
    ./rebar get-deps
    BITCASK_IO_MODE="nif" ./rebar skip_deps=true eunit
    ./rebar eunit skip_deps=true
Ignore log/ directory
Merge pull request #179 from basho/feature/tools.mk-0.5.5

Update tools.mk to v0.5.5

Reviewed-by: reiddraper