Save md5 correctly when jhbuildPath doesn't exist yet

Save md5 correctly when jhbuildPath doesn't exist yet
https://bugs.webkit.org/show_bug.cgi?id=120548

Patch by Nick Diego Yamane <nick.yamane@openbossa.org> on 2013-09-02
Reviewed by Gustavo Noronha Silva.

After r152605, Md5 for jhbuild files are saved before the update
process, this prevents the script to restart update from scratch
when initial checkouts fail. However it causes an issue when builddir
(or builddir/Dependencies) doesn't exist yet. In that case the
saveJhbuildMd5 function fails to create md5 files.
This patch adds a checking for the jhbuildPath and creates it if
necessary before trying to open the md5 files.

* Scripts/update-webkit-libs-jhbuild:
(saveJhbuildMd5):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@154951 268f45cc-cd09-0410-ab3c-d52691b4dbfc