Bump minimum Armadillo version to 9.800.

[zoq]

As discussed here #315 we like to bump the minimum Armadillo version to 9.800. I think once merged we should do another release 2.18.0.

[conradsnicta]

The index.md on the ensmallen website would also need to be changed to reflect the updated version requirement:
https://github.com/mlpack/ensmallen.org/blob/master/index.md

[mlpack-bot]

Second approval provided automatically after 24 hours. 👍

[rcurtin]

When this is merged, we should also bump it in mlpack, otherwise we can get a situation where mlpack's Armadillo version requirement is less than ensmallen's, so a user could feasible download mlpack, download Armadillo 8.400.0, think "I'm good to go!", then suddenly find out that the newest ensmallen (via the autodownloader) actually needs an even newer version.

[shrit]

It is strange the error says it is unable to find Armadillo on windows
As @rcurtin said, we need to update it on the mlpack side as well. Maybe do that before merging this one?

[zoq]

I'll fix the windows build.

[zoq]

The auto-downloader downloads the ensmallen master branch? If that is the case, we should change that behavior, it should always download the latest stable version. Anyway, I'll open another PR on mlpack. With the release of another ensmallen, I also open a second PR to update ensmallen.org.

[rcurtin]

The auto-downloader downloads the ensmallen master branch? If that is the case, we should change that behavior, it should always download the latest stable version.

Right, it just downloads the latest stable version. So, we should just merge mlpack/mlpack#3043 before releasing another version of ensmallen. 👍