2018-03-29, Version 4.9.1 'Argon' (Maintenance)

Notable changes:

No additional commits.

Due to incorrect staging of the upgrade to the GCC 4.9.X compiler, the
latest releases for PPC little endian were built using GCC 4.9.X
instead of GCC 4.8.X. This caused an ABI breakage on PPCLE based
environments. This has been fixed in our infrastructure and we are
doing this release to ensure that the hosted binaries are adhering to
our platform support contract.

PR-URL: https://github.com/nodejs/node/pull/19681