Handle possible duplication of constraints in VersionRange

[keshav-space]

Eliminate the possibility of constraints duplication in VersionRange and add some tests to validate the same.

>>> from univers.version_range import NginxVersionRange
>>> range=NginxVersionRange.from_native("1.5.0+, 1.4.1+, 1.4.0+")
>>> str(range)
'vers:nginx/>=1.4.0|>=1.4.1|<1.5.0|<1.5.0|>=1.5.0'

Context: #42 (comment)