only try fc-cache reset on linux

fc-cache doesn't exist on osx :)

install.sh

@@ -18,8 +18,10 @@ fi
 eval $find_command | xargs -0 -I % cp "%" "$font_dir/"
 
 # Reset font cache on Linux
-if [[ -n `which fc-cache` ]]; then
-  fc-cache -f $font_dir
+if [[ `uname` != 'Darwin' ]]; then
+  if [[ -n `which fc-cache` ]]; then
+    fc-cache -f $font_dir
+  fi
 fi
 
 echo "All Powerline fonts installed to $font_dir"