add full cross compilation in travis (#963)

* add full cross compilation in travis

* reduce the travis build matrix

* disable cross build for plan9 and nacl

travis/cross_build.sh

@@ -0,0 +1,5 @@
+#!/bin/bash
+
+if [[ "$TRAVIS_GO_VERSION" =~ ^1.\12\. ]] && [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
+    /tmp/gox/gox -build-lib -all -os '!plan9 !nacl'
+fi

travis/install.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+
+set -e
+
+if [[ "$TRAVIS_GO_VERSION" =~ ^1.\12\. ]] && [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
+    git clone https://github.com/dgsb/gox.git /tmp/gox
+    pushd /tmp/gox
+    git checkout new_master
+    go build ./
+    popd
+fi