- update bootstrap script to account for hex_core in src/vendored

- Vendor in hex_core at v0.6.8
 - Rename checksum to inner_checksum and deprecate in favor of new outer_checksum key/val
 - Change warn_vsn_once/0 to warn_vsn_once/1 so we can give a proper error message and
   take appropriate action.
 - Update rebar.lock to 1.2.0 format with continued support third-party
   that still may make use of inner checksum via continued persistence of
   inner checksums.
 - Changed app info tuple (package representation) to support inner and
   outer hashes for backwards compat support (see above)

version 0.5.1 is a maintenance release of hex_core specifically for
 rebar3 which contains a configuration update. Prior to v0.5.1 if no
 repo_organization key was set this could result in a function clause
 error. The behavior is to now set repo_organization to undefined in
 this case.

- vendor in hex_core at v0.5.0
  - Change where repo_name should be the org and not the parent
  - Changed rebar_utils:url_append_path/2 to not explicitly add a '?',
    this is returned in the Query chunk by http_uri:parse/1 (e.g., "?foo=bar")
  - update organization_merging test to expect the sub-repo as the repo_name
  - Add tests for rebar_utils:url_append_path/2
  - Stop referencing/setting "organization" in config and use new
     organization settings (api_repository and repo_organization)
  - Do not set (assume) the read key is valid for any/every repo
  - Set repo_organization and api_repository to org
  - Update tests to check for new config opts