In order for Slither to have support for SVG files, you need to have ImageMagick installed. Here are some installation instructions for common platforms.
Download and install the correct binary from ImageMagick's binaries page.
If you have homebrew installed, the easiest way is to run brew install imagemagick in your terminal.
If you have MacPorts installed, you can run sudo port install ImageMagick to install ImageMagick.
Otherwise, follow the instructions at ImageMagick's binaries page
Run sudo aptitude update && aptitude install imagemagick.
Run sudo apt-get install imagemagick.
Use your package manager to install ImageMagick, if has it.
Otherwise, try ImageMagick's binaries page.