Add clean target to ruby-hiera-eyaml/debian/rules

Add a `clean` target to the `debian/rules` file for the `ruby-hiera-eyaml`
package. This prevents the following error, which occurs when
`--buildsystem=ruby --with ruby` is specified for the `clean` target when
running `dh`:

    vagrant@packager:~/packager$ ./build_source.py pkg/ruby-hiera-eyaml/
    => creating build directory
    => fetching tarball
    => expanding tarball
    => copy new debian directory
    => debuild...
    dpkg-buildpackage -rfakeroot -d -us -uc -S
    dpkg-buildpackage: export CFLAGS from dpkg-buildflags (origin: vendor): -g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat -Wformat-security
    dpkg-buildpackage: export CPPFLAGS from dpkg-buildflags (origin: vendor): -D_FORTIFY_SOURCE=2
    dpkg-buildpackage: export CXXFLAGS from dpkg-buildflags (origin: vendor): -g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat -Wformat-security
    dpkg-buildpackage: export FFLAGS from dpkg-buildflags (origin: vendor): -g -O2
    dpkg-buildpackage: export LDFLAGS from dpkg-buildflags (origin: vendor): -Wl,-Bsymbolic-functions -Wl,-z,relro
    dpkg-buildpackage: source package ruby-hiera-eyaml
    dpkg-buildpackage: source version 2.0.2-1
    dpkg-buildpackage: source changed by Stig Sandbeck Mathisen <ssm@debian.org>
    dpkg-source --before-build ruby-hiera-eyaml-2.0.2
    dpkg-source: info: patches are not applied, applying them now
    dpkg-source: info: applying 0001-Do-not-require-rubygems.patch
    fakeroot debian/rules clean
    dh clean --buildsystem=ruby --with ruby
    dh: unable to load addon ruby: Can't locate Debian/Debhelper/Sequence/ruby.pm in @INC (@INC contains: /etc/perl /usr/local/lib/perl/5.14.2 /usr/local/share/perl/5.14.2 /usr/lib/perl5 /usr/share/perl5 /usr/lib/perl/5.14 /usr/share/perl/5.14 /usr/local/lib/site_perl .) at (eval 3) line 2.
    BEGIN failed--compilation aborted at (eval 3) line 2.

    make: *** [clean] Error 2
    dpkg-buildpackage: error: fakeroot debian/rules clean gave error exit status 2
    debuild: fatal error at line 1350:
    dpkg-buildpackage -rfakeroot -d -us -uc -S failed
    vagrant@packager:~/packager$

See also the discussion in https://github.com/alphagov/packager/pull/46