Bump the gitaly version to 1.7.0

Changelog from gitaly:

** v1.7.0

**** Added
- Add an RPC that allows repository size to be reduced by bulk-removing internal references
  https://gitlab.com/gitlab-org/gitaly/merge_requests/990

** v1.6.0

**** Other
- Clean up invalid keep-around refs when performing housekeeping
  https://gitlab.com/gitlab-org/gitaly/merge_requests/992

** v1.5.0

**** Added
- Add tls configuration to gitaly golang server
  https://gitlab.com/gitlab-org/gitaly/merge_requests/932

**** Fixed
- Fix TLS client code on macOS
  https://gitlab.com/gitlab-org/gitaly/merge_requests/994

**** Other
- Update to latest goimports formatting
  https://gitlab.com/gitlab-org/gitaly/merge_requests/993

** v1.4.0

**** Added
- Link and Unlink RPCs
  https://gitlab.com/gitlab-org/gitaly/merge_requests/986

** v1.3.0

*** Other
- Remove unused bridge_exceptions method
  https://gitlab.com/gitlab-org/gitaly/merge_requests/987
- Clean up process documentation
  https://gitlab.com/gitlab-org/gitaly/merge_requests/984

GITALY_SERVER_VERSION

-1.2.0
+1.7.0