updates link to REPL

Review URL: https://github.com/google/traceur-compiler/pull/597

Review URL: https://github.com/google/traceur-compiler/pull/599

Closes #599, closes #597.
Build intermediate traceur.js with last-good node front

./traceur combines bin/traceur.js with src/node code. While self-compiling, we use last-known good bin/traceur.js
from git. We must also use last-known good src/node.
Build dep.mk in evalCodeUnit