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[skip ci]

MRG: fix use of env_vars.sh for docker container

Path of env_vars.sh file should be relative to the repository, but when
we run in docker, the PWD is /, not /io, which is the repository root.

Strip binaries on non-OSX builds

Osx reliability improvements

Updated arb to 2.12.0

MRG: Lift branch limitations

Allow users to test from topic branches.

* Don't update homebrew

Resolution option 2-1 for https://github.com/matthew-brett/multibuild/issues/116

delocate-listdeps is not required and can take a several minutes for
heavyweight wheels.

* Add config.pre to override envvars used in `library_builders.sh`
Fixes part 2 of https://github.com/matthew-brett/multibuild/issues/116

* rename config_pre.sh to env_vars.sh as suggested

Add notes clarifying what to check out
Addresses gh-38.

* install.ps1 causes https://github.com/appveyor/ci/issues/2023 in Miniconda 3.6

#111 dropped support

It appears that is_function actually
calls the function (debugged). Why
that happens is a mystery to me, but
this should ensure that the spinner
runs as it is supposed to rather than
waiting until pre_build is complete.

See https://github.com/travis-ci/travis-ci/issues/8703#issuecomment-348866918

* Allow to specify a custom $CONFIG_PATH

* need to also pass  to the docker container