configury: keep gnulib revision under version control.

It's important to keep the gnulib revision number used by any
particular checkout of luaposix under version control rather than
hoping that whatever the current HEAD is will work.
* .gitignore: Remove gnulib.
* gnulib: Add current master revision.
Merge pull request #50 from gvvaughan/pull-request/keep-gnulib-revision-under-version-control

configury: keep gnulib revision under version control.