Release v5.1.6

• Update README.md by @BitcoinMitchell in #49
• Bump slevomat/coding-standard from 7.1 to 7.2.0 by @dependabot in #51
• Bump smarty/smarty from 3.1.44 to 3.1.45 by @dependabot in #54
• Bump slevomat/coding-standard from 7.2.0 to 7.2.1 by @dependabot in #52
• Bump guzzlehttp/guzzle from 7.4.2 to 7.4.4 by @dependabot in #55
• Small improvements by @BitcoinMitchell in #57
• Bumped module version to 5.1.6 by @BitcoinMitchell in #58

Full Changelog: v5.1.5...v5.1.6