Add clean target to Ruby gem packages

65f7983 added a `clean` target to `debian/rules` for the
`ruby-hiera-eyaml` package to prevent build errors.

This commit does the same, but for the `ruby-highline`, `ruby-trollop`
and `ruby-hiera-eyaml-gpg` packages.
Merge pull request #65 from alphagov/prevent-build-dependency-errors

Add clean target to Ruby gem packages