CI: 3rd attempt: trying to incorporate .net sdk into travis build

.travis.yml

@@ -5,7 +5,6 @@ cache:
     - $DOCKER_TRAVIS_CI_CACHE_DIR
 sudo: required
 language: cpp
-dotnet: 2.1.4
 services:
   - docker
 env:
@@ -86,9 +85,15 @@ matrix:
       osx_image: xcode8.3
       # Note: Apple Clang does not support OpenMP
       env: Z3_BUILD_TYPE=RelWithDebInfo USE_OPENMP=0 DOTNET_BINDINGS=0
+
+matrix:
+  include:
+    - language: csharp
+      dotnet: 2.1.4
+      script:
+        # Add at least one dotnet command invocation to activate the build env.
+        - dotnet restore
 script:
-  # Add at least one dotnet command invocation to activate the build env.
-  - dotnet restore
   # Use `travis_wait` when doing LTO builds because this configuration will
   # have long link times during which it will not show any output which
   # TravisCI might kill due to perceived inactivity.