Oops. Revert "Bootstrap with a custom babushka tarball with $TARBALL_URI."

This won't work: it's the repo itself that has to be customisable,
not the tarball, which is only used to bootstrap the install.

This reverts commit 772d0ae653dcaa73255a3fd6671fa53c60f85556.
Format the babushka invocation over multiple lines.
Clarify and add locking examples to 'installing' / 'versions'.
Add a $BABUSHKA_REPO env var, to customise the install repo.
Document installing from a custom repo using $BABUSHKA_REPO.
Merge branch 'custom-uri' again.

This time the install repo, not the tarball, is customised.

closes gh-7