Productmd 1.4

• Fix loading variants from legacy composeinfo.
• Fix sorting composes
• Compose: scan all subdirs under compose_path for metadata.
• Add Python 3.6 on Travis
• tests: add tests for ComposeInfo.get_variants()
• tests: composeinfo variant arches are sets
• composeinfo: py3 support for sort in get_variants()
• composeinfo: py3 support for iter in get_variants()
• composeinfo: check variant arches as a set
• composeinfo: fix arch kwarg handling in get_arches()
• Configure bztar with setup.cfg
• Remove requirements.txt
• Include requirements.txt in tarball
• Move %license definition just before %files
• Remove builder.test from releasers.conf
• Install deps with setup.py on Travis