  Using deep and high-cadence gamma-ray burst (GRB) afterglow data from RATIR,
we observe a sharp and achromatic light curve break 12.6 days after the GRB,
accompanied by an approximately achromatic bump. Fitting of the optical, NIR,
and X-ray data suggest a very narrow (2 degree) jet which remains collimated at
late-time. We argue that the sharp light curve bump suggests an edge brightened
jet, perhaps emitting only during a brief period of lateral jet expansion. The
lightcurve also exhibits a gradual spectral evolution lasting $>10$ days. The
evolution of the flux can be modeled as $\textrm{Flux} \sim \big(\frac{t}{[20
\textrm{days}]}\big)^\alpha \big(\frac{\lambda}{[800 \textrm{nm}]}\big)^\beta$,
with a temporal slope $\alpha=-0.956 \pm 0.003$ and a gradually time-varying
spectral slope $\beta = (0.60 \pm 0.07)+(0.26 \pm 0.06)
\textrm{log}\big(\frac{t}{20 \rm{days}}\big)$.
