If you are getting an error because git is not installed, we, that sucks. I was under the impression Composer would be smart enough to get the code as tarball in such a case. Though perhaps it normally is, and simply cannot do this in your case, as you are likely specifying dev-master (which refers to the master branch on git) to be installed.
Two things I suggest trying, that might get around this issue:
* add the --prefer-dist flag at the end of your Composer install command when running it. This will get packages as tarballs when available.
* rather then using "dev-master" as version, use "*"
In any case, I highly suspect that this will not occur when specifying 1.9 once it has been released. You can try installing version 1.0 the SubPageList extension  and see if you run into the same issue or not.