Fix JRuby install on systems with non-BSD `sed`

There doesn't seem to be a way to construct a sed invocation for
in-place editing that is compatible with both BSD and GNU sed.

http://stackoverflow.com/q/5694228/11687

Fixes #475

bin/ruby-build

@@ -519,7 +519,8 @@ install_jruby_launcher() {
 fix_jruby_shebangs() {
   for file in "${PREFIX_PATH}/bin"/*; do
     if [ "$(head -c 20 "$file")" = "#!/usr/bin/env jruby" ]; then
-      sed -i '' -E "1s:.+:#\!${PREFIX_PATH}/bin/jruby:" "$file"
+      sed -i.bak -E "1s:.+:#\!${PREFIX_PATH}/bin/jruby:" "$file"
+      rm "$file".bak
     fi
   done
 }