need to use mono 5.4.0

vagrant · vagrant · commit a2934de611ea · 2019-01-24T15:14:35.000-08:00
msbuild version packaged in 5.4.0 should be recent enough for the build
to work
diff --git a/.travis.yml b/.travis.yml
@@ -1,7 +1,7 @@
 language: csharp
 dist: xenial
 mono:
-  - 5.2.0
+  - 5.4.0
 # needed to make travis run the unit tests
   - 5.18.0
 solution: "./OptimizelySDK.Travis.sln"
