Ignore .fls files in LaTeX projects

From the [man page for latexmk](http://manpages.ubuntu.com/manpages/raring/man1/latexmk.1L.html):

> [...]  the  `-recorder`  option  with latex and pdflatex.  In (most)
> modern versions of these programs, this results  in  a  file  of
> extension  **.fls**  containing  a  list  of  the  files  that these
> programs have read and written.

Latexmk specifies this option in its latex commands and so produces temporary files with this extension in latex projects.