Improve LaTeX template

[devmotion]

I had to spend a lot of time on debugging issues with the Whedon bot in JuliaCon/proceedings-review#109, until I noticed that apparently the remaining culprit was the acknowledgement environment defined in juliacon.cls that I had used. So I replaced it with a simpler definition that does not involve the sans-serif font family and could finally compile the submission successfully (which, IMO, consists of a simple LaTeX file with a small number of packages).

During my online debugging session, I also made some other changes which I included in this PR as well. Maybe they are useful more generally, otherwise I'll split or reduce the PR.

To summarize:

• Commandline flags for latexmk can be dropped if the options are added to the .latexmkrc config file
• latexsym is removed as it should not be used in LaTeX2e documents (https://tex.stackexchange.com/questions/495254/latexsym-and-url-with-tikzposter-warning-about-size#comment1250328_495254)
• Use RequirePackage in juliacon.cls as it seems to be the convention in class files (https://www.mail-archive.com/[email protected]/msg77845.html)
• Replace the obsolete times package (https://www.ctan.org/pkg/times) with mathptmx: To obtain the same font style and in particular to not change the math font, the roman font is set with \renewcommand{\rmdefault}{ptm} instead of usepackage{mathptmx} (explained in the package documentation, section 9.1: https://ftp.acc.umu.se/mirror/CTAN/macros/latex/required/psnfss/psnfss2e.pdf)
• The definition of the acks environment is simplified and made whedon-compatible