Farmer

* Update requests dependency.

* `farmer` can now be installed on Python 3.7 without errors.
* Fixed six versioning issue that caused host list issues on Linux for some commands.

Added

* `logdna`: Added `logdna` command to export lines from [LogDNA](https://logdna.com/).

Added

* Accept configuration from environment variables (`FARMER_<key>`).

Changed

* Unite `--version` and `version` output.

Added

* `deploy`: Add `-b`/`--branch` argument to deploy specific Git reference. ([6])

Changes

* `config`: Direct user to portal for API token.

Fixes

* `config`: Handle empty YAML files. ([5])