Add Bazel CI configuration

This was previously present in
https://github.com/bazelbuild/continuous-integration, but the Bazel team
asked us to move it to our repo. It will be helpful if we need to change
the build configuration, eg to add new platforms.

.bazelci/presubmit.yml

+---
+platforms:
+  ubuntu1404:
+    build_targets:
+    - "..."
+    test_targets:
+    - "..."
+  ubuntu1604:
+    build_targets:
+    - "..."
+    test_targets:
+    - "..."
+  macos:
+    build_targets:
+    - "..."
+    test_targets:
+    - "..."