Drop Ubuntu 16.04 support; Trying 20.04

An issue with the ntp recipe breaks the Ubuntu 16.04 build:

```
Ran /bin/systemctl --system restart apparmor returned 1
```

See https://travis-ci.org/github/sealuzh/cwb-chef-repo/builds/702873714

Maybe related to sous-chefs/ntp#177

.travis.yml

@@ -18,7 +18,7 @@ env:
   jobs:
   - INSTANCE=ubuntu-1804
   # Add on demand:
-  - INSTANCE=ubuntu-1604
+  - INSTANCE=ubuntu-2004
 
 before_install:
   - cd cookbooks/cwb-server

cookbooks/cwb-server/.kitchen.dokken.yml

@@ -18,13 +18,12 @@ driver:
     - RUN locale-gen en_US.UTF-8
 
 platforms:
-- name: ubuntu-16.04
-  driver:
-    image: dokken/ubuntu-16.04
-
 - name: ubuntu-18.04
   driver:
     image: dokken/ubuntu-18.04
+- name: ubuntu-20.04
+  driver:
+    image: dokken/ubuntu-20.04
 
 transport:
   name: dokken

cookbooks/cwb-server/.kitchen.yml

@@ -13,8 +13,8 @@ verifier:
 
 platforms:
 # Node attributes: cat /tmp/kitchen/nodes/default-ubuntu-1604.json
-  - name: ubuntu-16.04
-  - name: ubuntu-18.04
+- name: ubuntu-18.04
+- name: ubuntu-20.04
 
 suites:
   - name: default

cookbooks/cwb-server/Makefile

@@ -1,5 +1,5 @@
 ### Configurable Variables
-# Example: `make all_tests INSTANCE=ubuntu-1604`
+# Example: `make all_tests INSTANCE=ubuntu-2004`
 INSTANCE=default-ubuntu-1804
 LOG_LEVEL=info # or debug
 

cookbooks/cwb-server/README.md

@@ -6,8 +6,9 @@ Installs and configures the Cloud WorkBench server.
 
 ## Supported Platforms
 
-* Ubuntu 18.04 64bit (tested 2019-01-02)
-* Ubuntu 16.04 64bit (tested 2019-01-02)
+* Ubuntu 18.04 64bit (manually tested 2019-01-02 and monthly tested through CI build)
+* Ubuntu 20.04 64bit (monthly tested through CI build)
+* NOTE: Ubuntu 16.04 fails due to a an [issue](https://github.com/chef-cookbooks/ntp/issues/177) with the ntp recipe
 
 Ensure that the default locale `en_US.UTF-8` exists (e.g., install via `sudo locale-gen en_US.UTF-8`) or configure via the attribute `cwb-server.system.locale`