Migrate away from using direct=false process types

[edmorley]

The direct=false process type mode has been removed as of Buildpack API version 0.9.

This buildpack currently uses direct=false mode, and so before we can update to Buildpack API 0.9 (and thus libcnb 0.12.0), we must migrate the buildpack to direct=true mode (and explicitly wrapping the command in bash).

An attempt at this was made in #150, however, it turns out we're blocked on cnb-shim adding support first, so that PR was closed.

Once the cnb-shim issue is resolved, we should revisit #150.

See:
#150 (comment)
heroku/cnb-shim#74

GUS-W-13119392.

[edmorley]

Once the cnb-shim issue is resolved, we should revisit #150.

cnb-shim is now broken for other reasons (see heroku/cnb-shim#69 (comment)), so cnb-shim being modernised is no longer a blocker for upgrading procfile to newer buildpack API and direct=true.