Fix package builds using git-buildpackage

With these changes, it's possible to build the packages using git-buildpackage:

1. tag the master branch for release using a `0.YYYYMMDD` version number: `git tag v0.YYYYMMDD`
2. merge the changes into the debian branch: `git checkout debian && git merge v0.YYYYMMDD`
3. bump the version number in `debian/changelog` to `0.YYYYMMDD-x`: `dch -i`
4. build the package: `git buildpackage -us -uc`

Note that I've assumed an upstream version numbering of 0.YYYYMMDD for the pre-release versions. This could be changed of course, but we should probably avoid having any ~ in version numbers since it's a reserved character in git. Also, we need to make sure that the pre-release version numbers are all smaller than `1.0` using the standard Debian sorting algorithm.
Thanks for helping us get setup @fmarier!  We do still need need options-ssl.conf though... it is under letsencrypt/client/options-ssl.conf.