Handle malformed tags

[reubano]

ongeza crashes if there are any non semver compliant tags present:

$ ongeza -tn
Traceback (most recent call last):
  File "/Users/reubano/Library/Python/3.6/bin/ongeza", line 17, in <module>
    main.run()
  File "/Users/reubano/Library/Python/3.6/lib/python/site-packages/ongeza/main.py", line 180, in run
    new_version = ongeza_project(project)
  File "/Users/reubano/Library/Python/3.6/lib/python/site-packages/ongeza/main.py", line 127, in ongeza_project
    new_version = project.ongeza(args.ongeza_type)
  File "/Users/reubano/Library/Python/3.6/lib/python/site-packages/ongeza/__init__.py", line 240, in ongeza
    if new_version in set(self.versions):
  File "/Users/reubano/Library/Python/3.6/lib/python/site-packages/ongeza/__init__.py", line 131, in versions
    versions = (t.lstrip('v') for t in self.tags)
  File "/Users/reubano/Library/Python/3.6/lib/python/site-packages/ongeza/git_utils.py", line 102, in tags
    return sorted(tags, key=cmp_to_key(compare))
  File "/Users/reubano/Library/Python/3.6/lib/python/site-packages/ongeza/git_utils.py", line 101, in <lambda>
    compare = lambda x, y: semver.compare(x.lstrip('v'), y.lstrip('v'))
  File "/Users/reubano/Library/Python/3.6/lib/python/site-packages/semver.py", line 187, in compare
    v1, v2 = parse(ver1), parse(ver2)
  File "/Users/reubano/Library/Python/3.6/lib/python/site-packages/semver.py", line 50, in parse
    raise ValueError('%s is not valid SemVer string' % version)
ValueError: 0.3 is not valid SemVer string