Merge pull request #61 from ryanfb/make_rbenv_install_dir

Some ruby-build definitions expect the target (parent) directory to exist

bin/ruby-build

@@ -168,6 +168,7 @@ remove_windows_files() {
 }
 
 build_package_copy() {
+  mkdir -p "$PREFIX_PATH"
   cp -R . "$PREFIX_PATH"
 }