go.tools/cmd/present: re-enable transitions
go.tools/cmd/present: re-enable transitions

They were disabled by mistake during the move to go.tools.

LGTM=dan.kortschak
R=golang-codereviews, dan.kortschak
CC=golang-codereviews
https://codereview.appspot.com/98440048

8limbs/gotools,webfd/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,d0f/go-zh.tools,8limbs/gotools,skyportsystems/go.tools,skyportsystems/go.tools,8limbs/gotools,bryanxu/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools