add ignore achemso bib file (acs-*.bib) to TeX.gitignore

Ignore acs-*.bib, an aux file generated by \documentclass{achemso} or by \usepackage{achemso}

http://ctan.org/pkg/achemso

achemso is a LaTeX package by Joseph Wright that anyone composing an article for the a Journal of the American Chemical Society would use, either for publications or for generating industry-standard bibliographic references.