don't delete ramdisk.tar.gz after extraction (palera1n#21)

deleting it causes undesired local changes in the git repo.

also added ramdisk.tar to gitignore so it doesn't show up as
an untracked file, though this one is less of an issue.

.gitignore

@@ -4,3 +4,4 @@ Linux/gaster
 work
 sshramdisk
 .irecovery
+other/ramdisk.tar

sshrd.sh

@@ -196,7 +196,7 @@ if [ "$oscheck" = 'Darwin' ]; then
     hdiutil resize -sectors min work/ramdisk.dmg
 else
     if [ -f other/ramdisk.tar.gz ]; then
-        gzip -d other/ramdisk.tar.gz
+        gzip -k -d other/ramdisk.tar.gz
     fi
 
     "$oscheck"/hfsplus work/ramdisk.dmg grow 300000000 > /dev/null