Update Snapshotter#take_snapshot! return type

Commit 6dade5f278c647b694a45153a74982240230d20d changed this from
returning a Hash to returning the return value of ActiveRecord's
`#save!` which happens to be a boolean, but failed to update the
documentation of the return type for this method. This commit updates
the documentation.

Change-Id: I65962efcabfc6197a65430b3d7618300e7e69f93