validate: Use blang/semver (vs. a local regular expression)
validate: Use blang/semver (vs. a local regular expression)

Avoid the incorrect:

  $ ./ocitools generate
  $ ./ocitools validate --path=.
  FATA[0000] 0.6.0-dev is not a valid version format, please read 'SemVer v2.0.0'.

by using an off-the-shelf SemVer library.  Benedikt's library is the
first hit on [1], [2] lists more known consumers than other semver
packages, and the license is MIT [3].

[1]: https://godoc.org/?q=semver
[2]: https://godoc.org/github.com/blang/semver
[3]: https://github.com/blang/semver/blob/v3.1.0/LICENSE

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

opencontainers/runtime-tools,hmeng-19/ocitools,wking/ocitools-v2,liangchenye/ocitools,opencontainers/runtime-tools,opencontainers/ocitools,wking/ocitools-v2,opencontainers/ocitools,hmeng-19/ocitools,liangchenye/ocitools