Composer conflict when trying to install spark installer

Following the instructions here I run

composer global require "laravel/spark-installer=~1.0"

This produces the following error:

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Conclusion: don't install laravel/spark-installer v1.1.1
    - Conclusion: don't install laravel/spark-installer v1.1.0
    - Conclusion: don't install laravel/spark-installer v1.0.4
    - Conclusion: don't install laravel/spark-installer v1.0.3
    - Conclusion: don't install laravel/spark-installer v1.0.2
    - Conclusion: remove guzzlehttp/guzzle 4.2.3
    - Installation request for laravel/spark-installer ~1.0 -> satisfiable by laravel/spark-installer[v1.0.0, v1.0.2, v1.0.3, v1.0.4, v1.1.0, v1.1.1].
    - Conclusion: don't install guzzlehttp/guzzle 4.2.3
    - laravel/spark-installer v1.0.0 requires guzzlehttp/guzzle ~6.0 -> satisfiable by guzzlehttp/guzzle[6.0.0, 6.0.1, 6.0.2, 6.1.0, 6.1.1, 6.2.0, 6.2.1, 6.2.2].
    - Can only install one of: guzzlehttp/guzzle[6.0.0, 4.2.3].
    - Can only install one of: guzzlehttp/guzzle[6.0.1, 4.2.3].
    - Can only install one of: guzzlehttp/guzzle[6.0.2, 4.2.3].
    - Can only install one of: guzzlehttp/guzzle[6.1.0, 4.2.3].
    - Can only install one of: guzzlehttp/guzzle[6.1.1, 4.2.3].
    - Can only install one of: guzzlehttp/guzzle[6.2.0, 4.2.3].
    - Can only install one of: guzzlehttp/guzzle[6.2.1, 4.2.3].
    - Can only install one of: guzzlehttp/guzzle[6.2.2, 4.2.3].
    - Installation request for guzzlehttp/guzzle (locked at 4.2.3) -> satisfiable by guzzlehttp/guzzle[4.2.3].

Any ideas how to resolve this?