Add ability to depend on latest PureScript release candidate via "unstable" version

[JordanMartinez]

Since setup-purescript is used across core, contrib, web, and node, we'll need a way for CI to use the latest release candidate when migrating to v0.15.0.

Changing the version to 'unstable' for PureScript should select the latest release candidate, so we don't have to update it again if a new release candidate is made.

[thomashoneyman]

#25 is related as well