Improve the config discovery.

Before this commit smith searched up the directory hierarchy until it
got to home before raising a ConfigNotFoundError exception. This looks
for /etc/smithrc and /etc/smith/config before raising an exception.
Replace tdb with gdbm.

tdb, lmdb, leveldb have all had installation problems so just go with
the simplist: gdbm. It really doesn't make much difference for this
case.
Bump version to 0.6.3.
Use semantic versioning specs in gemspec.