v0.20.0: Update versions (#258)

* Update version to v0.18.0

* Refactor module interface
Move `mnemonic` to public`

* Fix linter warnings
Add docs

* Fix source code formatting

* Remove 32bit builds for Appveyor

* Update rocksdb version

* FIx travis config for rustfmt

* Remove rust beta channel from travis CI

* Move to stable formatter

* TravisCI: allow failures on nightly

* Remove MacOS & update version for rocksdb