Allow specifying cmake generator.
diff --git a/.travis.yml b/.travis.yml
index c293742..79e11f0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -32,6 +32,12 @@
     env:
       TOOLCHAIN=NATIVE
       TARGET=x86_64-windows
+      CMAKE_GENERATOR="Visual Studio 14 2015 Win64"
+  - os: windows
+    env:
+      TOOLCHAIN=NATIVE
+      TARGET=x86_64-windows
+      CMAKE_GENERATOR="Visual Studio 15 2017 Win64"
   - os: linux-ppc64le
     compiler: gcc
     env: