minor change to make compatible with tcsh

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?
