Don't test on Ubuntu 14.04

Ubuntu 14.04 is about to be end-of-life and Bazel CI will stop supporting it shortly afterwards.

Context: https://groups.google.com/d/msg/bazel-dev/_D6XzfNkQQE/8TNKiNmsCAAJ

.bazelci/presubmit.yml

diff --git a/.bazelci/presubmit.yml b/.bazelci/presubmit.yml
index 45b7c06..7b2bba3 100644
--- a/.bazelci/presubmit.yml
+++ b/.bazelci/presubmit.yml
@@ -1,9 +1,9 @@
 ---
 platforms:
-  ubuntu1404:
+  ubuntu1604:
     test_targets:
     - "//:all_tests"
-  ubuntu1604:
+  ubuntu1804:
     test_targets:
     - "//:all_tests"
   rbe_ubuntu1604: