Just 1.6

• add elixir plugs link
• add otp19.0 to travis
• update gpb/rebar3_gpb_plugin
• run elvis on travis-ci
• add elvis.
• Histogram constructors now accept buckets specs.
  {linear, Start, Step, Count} and {exponential, {Start, Factor, Count}
• rename prometheus_buckets generators
• Cleanup, and refactor standard metric constructors