Use system() with silent to avoid visual artifact

As mentioned in the README if we use an `updatetime` too low we could experience some glitches. To prevent this in recent version of vim [7.4.427](http://ftp.vim.org/vim/patches/7.4/7.4.427) every call to `system` should be done using `silent` if the command do not require the user input, as in the case of vim-gitgutter.