Remove Bintray from Spaces

[keilw]

• Maven POM: add OSSH snapshot repository, see https://central.sonatype.org/pages/apache-maven.html

As JCenter will remain it can stay at the moment, ans Spaces does not have a CI server yet.

See unitsofmeasurement/uom-parent#24

[somayaj]

Hi, would like to work on this. if you could provide some background and task description. Would like to contribute, thx