Update rocket.chat

Changes:

- https://github.com/RocketChat/Docker.Official.Image/commit/7b7930c: Adjust RC-skipping code to also skip Beta releases (https://github.com/RocketChat/Docker.Official.Image/pull/74)
- https://github.com/RocketChat/Docker.Official.Image/commit/641319b: Update to 1.0.0-beta.2
- https://github.com/RocketChat/Docker.Official.Image/commit/154abb6: Update to 1.0.0-beta.1
- https://github.com/RocketChat/Docker.Official.Image/commit/d68490a: Update to 1.0.0-beta.0
- https://github.com/RocketChat/Docker.Official.Image/commit/2b78256: Add volume to docker-compose (https://github.com/RocketChat/Docker.Official.Image/pull/72)
- https://github.com/RocketChat/Docker.Official.Image/commit/3c02990: update Docker's "MAINTAINER" instruction (https://github.com/RocketChat/Docker.Official.Image/pull/70)

library/rocket.chat

@@ -2,4 +2,4 @@ Maintainers: Rocket.Chat Image Team <[email protected]> (@RocketChat)
 GitRepo: https://github.com/RocketChat/Docker.Official.Image.git
 
 Tags: 0.74.3, 0.74, 0, latest
-GitCommit: 1a592ab2c5ca235e6ac2fa265f65d294490c044c
+GitCommit: 7b7930c18ab19ec9848d643ff6929e2860680017