Add changelog and version updates for Zulip Server 1.9.1.

The actual changes in 1.9.1 are already present in master.

docs/conf.py

@@ -54,7 +54,7 @@ author = 'The Zulip Team'
 # The short X.Y version.
 version = '1.9+git'
 # The full version, including alpha/beta/rc tags.
-release = '1.9.0+git'
+release = '1.9.1+git'
 
 # This allows us to insert a warning that appears only on an unreleased
 # version, e.g. to say that something is likely to have changed.

docs/overview/changelog.md

@@ -7,6 +7,19 @@ All notable changes to the Zulip server are documented in this file.
 This section lists notable unreleased changes; it is generally updated
 in bursts.
 
+### 1.9.1 -- 2018-11-30
+
+This release is primarily intended to improve the experience for new
+Zulip installations; it has minimal changes for existing servers.
+
+- Added support for getting multi-domain certificates with setup-certbot.
+- Improved various installer error messages and sections of the
+  installation documentation to help avoid for common mistakes.
+- The Google auth integration now always offers an account chooser.
+- Fixed buggy handling of avatars in Slack import.
+- Fixed nginx configuration for mobile API authentication to access uploads.
+- Updated translation data, including significant new Italian strings.
+
 ### 1.9.0 -- 2018-11-07
 
 **Highlights:**

version.py

@@ -1,6 +1,6 @@
-ZULIP_VERSION = "1.9.0+git"
+ZULIP_VERSION = "1.9.1+git"
 LATEST_MAJOR_VERSION = "1.9"
-LATEST_RELEASE_VERSION = "1.9.0"
+LATEST_RELEASE_VERSION = "1.9.1"
 LATEST_RELEASE_ANNOUNCEMENT = "https://blog.zulip.org/2018/11/07/zulip-1-9-released/"
 
 # Bump the minor PROVISION_VERSION to indicate that folks should provision