Bump max_bazel_version to 0.20.0

After a few fixes, TensorFlow now builds with Bazel 0.20.0. This allows us to keep testing TF on Bazel CI, where bazel is already upgraded to 0.20.0

PiperOrigin-RevId: 224006905
diff --git a/configure.py b/configure.py
index 3a7999f..6c905a0 100644
--- a/configure.py
+++ b/configure.py
@@ -1565,7 +1565,7 @@
   # environment variables.
   environ_cp = dict(os.environ)
 
-  check_bazel_version('0.15.0', '0.19.2')
+  check_bazel_version('0.15.0', '0.20.0')
 
   reset_tf_configure_bazelrc()
   # Explicitly import tools/bazel.rc, this is needed for Bazel 0.19.0 or later