Added a forgotten shellescape

I realized I had forgotten to add a `shellescape` to the call of `synctex` when using MuPDF. @lervag, it would be nice if you could test this yourself. A command of this form:

![screenshot 2014-12-08 22 00 49](https://cloud.githubusercontent.com/assets/7765538/5347076/c9ba7706-7f25-11e4-9467-fe654da82df6.png)

works fine here:

![screenshot 2014-12-08 22 01 47](https://cloud.githubusercontent.com/assets/7765538/5347091/f027038c-7f25-11e4-936f-cba5976210ff.png)

**EDIT**: An absolute path within quotes passed to `synctex` works as well.