DOC: update the ROADMAP file

I already forgot to update it for dev20.
[RELEASE] Released version 1.5-dev21

Released version 1.5-dev21 with the following main changes :
    - MINOR: stats: don't use a monospace font to report numbers
    - MINOR: session: remove debugging code
    - BUG/MAJOR: patterns: fix double free caused by loading strings from files
    - MEDIUM: http: make option http_proxy automatically rewrite the URL
    - BUG/MEDIUM: http: cook_cnt() forgets to set its output type
    - BUG/MINOR: stats: correctly report throttle rate of low weight servers
    - BUG/MEDIUM: checks: servers must not start in slowstart mode
    - BUG/MINOR: acl: parser must also stop at comma on ACL-only keywords
    - MEDIUM: stream-int: implement a very simplistic idle connection manager
    - DOC: update the ROADMAP file