Increase .travis.yml consistency between repos

- aspnet/Universe#349
- minimize `dotnet` setup time; no need for caching

.travis.yml

@@ -10,6 +10,10 @@ addons:
     - libssl-dev
     - libunwind8
     - zlib1g
+env:
+  global:
+    - DOTNET_SKIP_FIRST_TIME_EXPERIENCE: true
+    - DOTNET_CLI_TELEMETRY_OPTOUT: 1
 mono:
   - 4.0.5
 os: