Set CI B2_CI_VERSION=0

[sdarwin]

Hi,

Going back some years, boostorg/boost-ci provided a new way to configure B2 variables, which takes effect using B2_CI_VERSION=1. For example:

(new) 'B2_LINKFLAGS': '-fuse-ld=gold'

instead of

(previous) 'B2_LINKFLAGS': 'linkflags=-fuse-ld=gold'

Now that most boost libraries have migrated, we'd like to standardize boost-ci onto B2_CI_VERSION=1.

You can opt-out by configuring B2_CI_VERSION=0 and then upgrade later.

This pull request sets B2_CI_VERSION=0, so CI will continue to work, using the previous method.

[jeking3]

I don't it matters much here since this repo does not have appveyor enabled at the moment.

[sdarwin]

does not have appveyor enabled

To be thorough, the update could be merged anyway. for future use.
Or delete appveyor.yml so that it's clear appveyor isn't enabled.