Bump docker container versions.

Dependencies

• [2] MJDIMD5B Improve documentation of local docker-compose setup.
• [3] NJNMO72S Add zcash.com submodule and update client to modern halogen.

Change contents

• replacement in DEVELOPMENT.md at line 27
  [2.314]→[2.314:373](∅→∅)
  − * `postgres/postgres:9.4`

  − * `electriccoinco/zcashd:v4.0.0`
  [2.314]

  [2.373]
  + * `postgres/postgres:latest`

  + * `electriccoinco/zcashd:v4.2.0`
• replacement in DEVELOPMENT.md at line 35
  [2.589]→[2.589:694](∅→∅)
  − docker pull nginx/nginx:lates

  − docker pull postgres/postgres:9.4

  − docker pull electriccoinco/zcashd:v4.0.0
  [2.589]

  [2.694]
  + docker pull nginx/nginx:latest

  + docker pull postgres/postgres:latest

  + docker pull electriccoinco/zcashd:v4.2.0