minor change to make compatible with tcsh

[Gullumluvl]

Hi,
I know that not so many people use the tcsh shell, but I inherited from it on my computer at work, and I noticed that one line in the ftplugin/latex-box/common.vim is not compatible:

line 252:

let s:uname = system('echo -n $(uname -s)')

Using backticks instead of $() would make it compatible for bash and tcsh.
Should I do a pull request?