Fixes lein_prefer script

[hallettj]

I was having some trouble getting the mori build scripts to work. There were two issues that are fixed by these changes.

The get_lein script produced an empty file when I ran it. I updated that script to use the wgetjs library. You still get a download utility that does not have dependencies on anything other than node (despite the name), but the testing burden is shifted to a third party. I also updated the download URL.

The lein_prefer script silently failed for my when which lein failed to find a lein executable.