Update trillian dependency (#281)

* Update trillian dependency to master

This removes calls to the trillian verifier and replaces them with calls to a new rekor verify package!
The `verify` package currently only verifies the signed log root.

Signed-off-by: Priya Wadhwa <priyawadhwa@google.com>

* Fix lint

Signed-off-by: Priya Wadhwa <priyawadhwa@google.com>

* Add verifier back in and update trillian images

Signed-off-by: Priya Wadhwa <priyawadhwa@google.com>

* Make verify private

Signed-off-by: Priya Wadhwa <priyawadhwa@google.com>

* Address code review comments

Signed-off-by: Priya Wadhwa <priyawadhwa@google.com>

* Roll back to trillian v1.3.13

Signed-off-by: Priya Wadhwa <priyawadhwa@google.com>

* Pin trillian to latest commit

Signed-off-by: Priya Wadhwa <priyawadhwa@google.com>