Errors processing characters in bibliography

[peastman]

A recent update to sphinx has broken building of PDFs for my project. I use sphinxcontrib-bibtex to include a bibtex bibliography. My bibliography includes the following entry.

@article{Best2012,
   author = {Best, Robert B. and Zhu, Xiao and Shim, Jihyun and Lopes, Pedro E. M. and Mittal, Jeetain and Feig, Michael and MacKerell, Alexander D.},
   title = {Optimization of the Additive CHARMM All-Atom Protein Force Field Targeting Improved Sampling of the Backbone $\phi$, $\psi$, and Side-Chain $\chi$1 and $\chi$2 Dihedral Angles},
   journal = {Journal of Chemical Theory and Computation},
   volume = {8},
   number = {9},
   pages = {3257-3273},
   year = {2012},
   type = {Journal Article}
}

Notice the Greek letters in the title, which are correctly specified in LaTeX format. Using sphinx 1.8.5 this builds correctly. But using 2.1.2 it fails with this error:

! Package inputenc Error: Unicode char \u8:φ not set up for use with LaTeX.

See the inputenc package documentation for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.9997 ...ing improved sampling of the backbone φ
                                                  , ψ, and side-chain χ1 a...