Support to rollback --alias.

[fujiwara]

refs #360

Support rollback --alias flag to specify the alias to rollback.

Now, lambroll finds the target version to rollback by the logic as below.

• List all versions with the lower version of the alias to rollback.
• Pick the highest version number from these.
  • but If the version has aliases, continue to the next lower version.

But, this algorithm has a problem. The selected version may be the previous version of the other aliases.

To solve the problem, we have to detect all older versions for each deployed alias.
But I have no good idea to solve it.

[fujiwara]

d7ee38a adds rollback --version flag. This flag can specify the version to rollback clearly.