Revert "$PREFIX_PATH can be specified as a relative path"

This reverts commit 20f7b5c.

Conflicts:

	bin/ruby-build

Author: sstephenson

bin/ruby-build

@@ -195,13 +195,11 @@ elif [ ! -e "$DEFINITION_PATH" ]; then
   fi
 fi
 
-if [ -z "$2" ]; then
+PREFIX_PATH="$2"
+if [ -z "$PREFIX_PATH" ]; then
   usage
 fi
 
-mkdir -p "$2"
-PREFIX_PATH="$(abs_dirname "$2")"
-
 if [ -z "$TMPDIR" ]; then
   TMP="/tmp"
 else