`--install-command` seems to be ignored

[murtyjones]

I added install-changed to a project that uses yarn, and set it up like so:

install-changed --install-command "yarn install --frozen-lockfile"

but npm install seems to always be used when I run the above command.

[murtyjones]

Seems like what's on master needs to be published to npm. This is what version 1.1.0 (latest release) looks like locally

[thdk]

See #15