travis: temporarily only build for linux/amd64 and darwin/amd64

Signed-off-by: Tibor Vass <tibor@docker.com>

.travis.yml

@@ -8,8 +8,12 @@ install:
 
 env:
   global:
-    - PLATFORMS="linux/amd64,linux/arm/v6,linux/arm/v7,linux/arm64,linux/s390x,linux/ppc64le"
+    # Temporarily disabling other platforms to keep development faster.
+    # - PLATFORMS="linux/amd64,linux/arm/v6,linux/arm/v7,linux/arm64,linux/s390x,linux/ppc64le"
+    # - CROSS_PLATFORMS=${PLATFORMS},darwin/amd64,windows/amd64
+    - PLATFORMS="linux/amd64"
+    - CROSS_PLATFORMS="${PLATFORMS},darwin/amd64"
     - PREFER_BUILDCTL="1"
 
 script:
-  - make binaries validate-all && TARGETPLATFORM="${PLATFORMS},darwin/amd64,windows/amd64" ./hack/cross
+  - make binaries validate-all && TARGETPLATFORM="${CROSS_PLATFORMS}" ./hack/cross