-cover causes races in Go 1.2

Make -covermode=atomic the default until we drop 1.2 support
Enable Go 1.2 in Travis

hack/install-std-race.sh

@@ -0,0 +1,28 @@
+#!/bin/bash
+
+# Copyright 2014 Google Inc. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# This script installs std -race on Travis (see https://code.google.com/p/go/issues/detail?id=6479)
+
+set -e
+
+if [ "${TRAVIS}" == "true" ]; then
+  GO_VERSION=($(go version))
+
+  if [ ${GO_VERSION[2]} \< "go1.3" ]; then
+    echo "Installing the -race compatible version of the std go library"
+    go install -a -race std
+  fi
+fi