Support upgrade in NIF library for Cuckoo miner
Support upgrade in NIF library for Cuckoo miner

These changes are meant to address test failure manifesting itself on
CI - not locally - when running EUnit tests as CT suite.

Extract of symptom on commit f8398f2 (full symptom at
https://github.com/aeternity/epoch/pull/312#issuecomment-341095112):
```
...
%%% aec_eunit_SUITE ==> aec_tx_pool_tests: OK

=ERROR REPORT==== 1-Nov-2017::10:59:37 ===

Error in process <0.4967.0> on node 'epoch_ct@travis-job-aeternity-epoch-295687632' with exit value:

{{badmatch,{error,{upgrade,"Upgrade not supported by this NIF library."}}},
...
```

Refs:
* http://erlang.org/doc/man/erl_nif.html#upgrade
* https://gitlab.com/barrel-db/erlang-rocksdb/blob/0.13.1/c_src/erocksdb.cc#L566

aeternity/epoch,aeternity/epoch,aeternity/epoch,aeternity/epoch