Update tensorboard nightly dependency to 1.15.x

Now that TensorBoard 1.14.0 has been released, our nightlies have moved
up to 1.15.x alphas.

PiperOrigin-RevId: 257662026

tensorflow/tools/pip_package/setup.py

@@ -95,7 +95,7 @@ else:
 if 'tf_nightly' in project_name:
   for i, pkg in enumerate(REQUIRED_PACKAGES):
     if 'tensorboard' in pkg:
-      REQUIRED_PACKAGES[i] = 'tb-nightly >= 1.14.0a0, < 1.15.0a0'
+      REQUIRED_PACKAGES[i] = 'tb-nightly >= 1.15.0a0, < 1.16.0a0'
     elif 'tensorflow_estimator' in pkg and '2.0' in project_name:
       REQUIRED_PACKAGES[i] = 'tensorflow-estimator-2.0-preview'
     elif 'tensorflow_estimator' in pkg: