Bump batteries to v4.9.0-rc2

Stream: batteries

[François G. Dorais]

Mathlib got bumped but Batteries did not. @Kim Morrison ?

[François G. Dorais]

batteries#847

[François G. Dorais]

The bump PR works fine so this is probably just an oversight.

[James Gallicchio]

i think it just means batteries compiles fine without modification on rc2?

[François G. Dorais]

But CI on batteries still uses v4.9.0-rc1.

[Kim Morrison]

Yes, this was intentional to save some time, as rc2 only made "user interface" changes, no langauge changes.

[François G. Dorais]

It needs to happen for batteries CI to work correctly and also for downstream projects who need to know when it's safe to update the toolchain.

[Kim Morrison]

What happened to batteries CI?

[François G. Dorais]

I had some weird inconsistent results. I thought the issue was that Batteries was still using rc1 for CI but it turns out it was a cache issue on my end. It's all fixed now!