Test cross-compiling within travis

Tests cross compiling by building on 1.6 of Go for Windows / Linux /
macOS

.travis.yml

@@ -10,8 +10,11 @@ go_import_path: github.com/kubernetes/kompose
 matrix:
   include:
     - go: 1.6
-      # Only build docs once
-      env: BUILD_DOCS=yes
+      env: 
+        # Only build docs once
+        - BUILD_DOCS=yes
+        # Test cross-compile as well
+        - CROSS_COMPILE=yes
     - go: 1.7
     - go: 1.8
 
@@ -22,6 +25,8 @@ install:
 
 script:
   - make test
+  # Only cross-compile once
+  - if [ "$CROSS_COMPILE" == "yes" ]; then make cross; fi
 
 after_success:
   # gover collects all .coverprofile files and saves it to one file gover.coverprofile
@@ -44,4 +49,4 @@ deploy:
     secure: "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"
   skip_cleanup: true
   on:
-    go: 1.8
+    go: 1.8