Add GITHUB_PAT to pass a token, to have a higher limit to pull from R repo
Add GITHUB_PAT to pass a token, to have a higher limit to pull from R repo

Travis CI set a limit to pull unless a GITHUB_PAT is passed. With
GITHUB_PAT we should have 5000/hour (vs. 60/hour).

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>

tensorflow/io,tensorflow/io,tensorflow/io,tensorflow/io,tensorflow/io,tensorflow/io,tensorflow/io