install: Update list of supported distros for installation.

This only changes the install script, not the docs, so it's pretty
low-profile; we'll update the docs after a bit more testing.

scripts/lib/install

@@ -77,7 +77,7 @@ export LC_ALL="en_US.UTF-8"
 apt-get install -y lsb-release
 os_release="$(lsb_release -sc)"
 case "$os_release" in
-    trusty|xenial|stretch) ;;
+    trusty|xenial|stretch|bionic) ;;
     *)
         set +x
         cat <<EOF
@@ -86,8 +86,9 @@ Unsupported OS release: $os_release
 
 Zulip in production is supported only on:
  - Debian 9 "stretch"
- - Ubuntu 14.04 LTS "trusty"
+ - Ubuntu 14.04 LTS "trusty" (deprecated)
  - Ubuntu 16.04 LTS "xenial"
+ - Ubuntu 18.04 LTS "bionic"
 
 For more information, see:
   https://zulip.readthedocs.io/en/latest/production/requirements.html