docs: Update docs to reflect the use of Ubuntu bionic in development.

docs/development/setup-vagrant.md

@@ -298,7 +298,7 @@ $ vagrant ssh
 You should see output that starts like this:
 
 ```
-Welcome to Ubuntu 14.04.1 LTS (GNU/Linux 4.4.0-21-generic x86_64)
+Welcome to Ubuntu 18.04.2 LTS (GNU/Linux 4.15.0-54-generic x86_64)
 ```
 
 Congrats, you're now inside the Zulip development environment!
@@ -311,7 +311,7 @@ provisioning failed and you should look at the
 Next, start the Zulip server:
 
 ```
-(zulip-py3-venv)vagrant@vagrant-ubuntu-trusty-64:/srv/zulip
+(zulip-py3-venv) vagrant@ubuntu-bionic:/srv/zulip
 $ ./tools/run-dev.py
 ```
 
@@ -457,7 +457,7 @@ From the window where run-dev.py is running:
 2016-05-04 18:33:13,330 INFO     127.0.0.1       GET     200  92ms /register/ (unauth via ?)
 ^C
 KeyboardInterrupt
-(zulip-py3-venv)vagrant@vagrant-ubuntu-trusty-64:/srv/zulip$ exit
+(zulip-py3-venv) vagrant@ubuntu-bionic:/srv/zulip$ exit
 logout
 Connection to 127.0.0.1 closed.
 christie@win10 ~/zulip
@@ -493,7 +493,7 @@ christie@win10 ~/zulip
 $ vagrant up
 $ vagrant ssh
 
-(zulip-py3-venv)vagrant@vagrant-ubuntu-trusty-64:/srv/zulip
+(zulip-py3-venv) vagrant@ubuntu-bionic:/srv/zulip
 $ ./tools/run-dev.py
 ```
 

docs/subsystems/migration-renumbering.md

@@ -36,7 +36,7 @@ showell@Steves-MBP ~/zulip (showell-topic) $ git status
 # On branch showell-topic
 nothing to commit, working directory clean
 
-(zulip-venv)vagrant@vagrant-ubuntu-trusty-64:/srv/zulip$ ./tools/test-backend
+(zulip-py3-venv) vagrant@ubuntu-bionic:/srv/zulip$ ./tools/test-backend
 <output skipped>
 DONE!
 ```
@@ -80,7 +80,7 @@ but I still need to run the tests to make sure there weren't any
 semantic conflicts with the new changes from master:
 
 ```
-(zulip-venv)vagrant@vagrant-ubuntu-trusty-64:/srv/zulip$ ./tools/test-backend
+(zulip-py3-venv) vagrant@ubuntu-bionic:/srv/zulip$ ./tools/test-backend
 
 <output skipped>
 
@@ -273,7 +273,7 @@ showell@Steves-MBP ~/zulip/zerver/migrations (showell-topic) $ git show 8022839f
 And then I run the tests and cross my fingers!!!:
 
 ```
-(zulip-venv)vagrant@vagrant-ubuntu-trusty-64:/srv/zulip$ ./tools/test-backend
+(zulip-py3-venv) vagrant@ubuntu-bionic:/srv/zulip$ ./tools/test-backend
 <output skipped>
   Applying zerver.0023_userprofile_default_language... OK
   Applying zerver.0024_realm_allow_message_editing... OK

templates/zerver/api/incoming-webhooks-walkthrough.md

@@ -230,7 +230,7 @@ After running the above command, you should see something similar to:
 Using `manage.py` from within the Zulip development environment:
 
 ```
-(zulip-py3-venv)vagrant@vagrant-ubuntu-trusty-64:/srv/zulip$
+(zulip-py3-venv) vagrant@ubuntu-bionic:/srv/zulip$
 ./manage.py send_webhook_fixture_message \
     --fixture=zerver/webhooks/helloworld/fixtures/hello.json \
     '--url=http://localhost:9991/api/v1/external/helloworld?api_key=<api_key>'
@@ -363,7 +363,7 @@ Once you have written some tests, you can run just these new tests from within
 the Zulip development environment with this command:
 
 ```
-(zulip-py3-venv)vagrant@vagrant-ubuntu-trusty-64:/srv/zulip$
+(zulip-py3-venv) vagrant@ubuntu-bionic:/srv/zulip$
 ./tools/test-backend zerver/webhooks/helloworld
 ```
 
@@ -412,7 +412,7 @@ stream name:
 To trigger a notification using this webhook, use
 `send_webhook_fixture_message` from the Zulip command line:
 
-    (zulip-py3-venv)vagrant@vagrant-ubuntu-trusty-64:/srv/zulip$
+    (zulip-py3-venv) vagrant@ubuntu-bionic:/srv/zulip$
     ./manage.py send_webhook_fixture_message \
         --fixture=zerver/tests/fixtures/helloworld/hello.json \
         '--url=http://localhost:9991/api/v1/external/helloworld?api_key=&lt;api_key&gt;'

zerver/webhooks/helloworld/doc.md

@@ -15,7 +15,7 @@ To trigger a notification using this webhook, use
 `send_webhook_fixture_message` from the Zulip command line:
 
 ```
-(zulip-py3-venv)vagrant@vagrant-ubuntu-trusty-64:/srv/zulip$
+(zulip-py3-venv) vagrant@ubuntu-bionic:/srv/zulip$
 ./manage.py send_webhook_fixture_message \
 > --fixture=zerver/tests/fixtures/helloworld/hello.json \
 > '--url=http://localhost:9991/api/v1/external/helloworld?api_key=<api_key>'