Test against several versions of Augeas

This PR tests with several versions of Augeas and fixes some tests that wouldn't pass on Augeas 0.10.0.

NB: tests fail on Augeas 1.2.0 because of bug #83, which requires PR #80 to be fixed. For this reason, Augeas 1.2.0 tests are marked as `allow_failures` for now and 1.1.0 is used as default version. This can be changed again when bug #83 is fixed (see https://github.com/raphink/augeasproviders/tree/dev/shellvar_var_action_seq_one_handler)