Switch off Travis container-based infrastructure

Something about running tests inside a docker container causes the tests to timeout

[skip appveyor]

.travis.yml

@@ -1,10 +1,14 @@
 language: csharp
-sudo: false
+sudo: required
 dist: trusty
 env:
   global:
     - DOTNET_SKIP_FIRST_TIME_EXPERIENCE: true
     - DOTNET_CLI_TELEMETRY_OPTOUT: 1
+addons:
+  apt:
+    packages:
+      - libunwind8
 mono: none
 os:
   - linux