Fix issue with incorrect tag generation

[ajpetersons]

When existing tags were not sorted, next tag was generated incorrectly. The program would assume minor version from a smaller major version in some cases. For example, if current largest known version was 1.0.0 and 0.0.1 was seen next, the program would increment patch version and produce the next tag as 1.0.2 instead of 1.0.1. This is fixed by only incrementing minor and patch versions if all. other version numbers match.

See #13 .