Fixes bug 926078 - updates stackwalker

The new stackwalker was updated in HG after we brought it into the
Socorro repo. This ports over those changes from https://hg.mozilla.org/users/tmielczarek_mozilla.com/minidump-stackwalk/rev/b99899ee43c7
Merge pull request #1582 from lonnen/926078-stackwalker

Fixes bug 926078 - updates stackwalker