ci: upgrade powershell

Author: felixfbecker

.travis.yml

@@ -1,13 +1,18 @@
 language: minimal
 
-dist: trusty
-addons:
-  apt:
-    sources:
-      - sourceline: deb [arch=amd64] https://packages.microsoft.com/ubuntu/14.04/prod trusty main
-        key_url: https://packages.microsoft.com/keys/microsoft.asc
-    packages:
-      - powershell
+dist: bionic
+
+before_install:
+  # Download the Microsoft repository GPG keys
+  - wget -q https://packages.microsoft.com/config/ubuntu/18.04/packages-microsoft-prod.deb
+  # Register the Microsoft repository GPG keys
+  - sudo dpkg -i packages-microsoft-prod.deb
+  # Update the list of products
+  - sudo apt-get update
+  # Enable the "universe" repositories
+  - sudo add-apt-repository universe
+  # Install PowerShell
+  - sudo apt-get install -y powershell
 
 script:
   - pwsh -c 'Import-Module ./PSSourcegraph.psd1'