Update: realpath method in util-path.js

1. DOUBLE_DOT_RE will match `a/b/c//../d` path correctly only if
replace `//` with `/` first
2.  `a///b/c` should be the path `a/b/c`
UPDATE: match multi slashes, not only double

Replace DOUBLE_SLASH_RE with MULTI_SLASH_RE. Now path like
`a////b////c` will be parsed to `a/b/c`correctly.
Merge pull request #1193 from wh1100717/master

Update: realpath method in util-path.js