do not apply detlas having the same version than the base line

[3nids]

fixes #72

[3nids]

not sure, it looks like there is an issue in the checking https://travis-ci.org/opengisch/pum/builds/651528596#L341 would require further investigation

[marioba]

I'm not against this, but it will probably break some user's workflows without real improvement. Maybe we can wait for a "major" release to implement this. Btw, the readme, the CLI help and maybe some docstrings need to be updated too.

[elemoine]

See https://github.com/opengisch/pum/pull/77 for a less aggressive solution :-)

[olivierdalang]

This would also help with https://github.com/opengisch/pum/issues/79