Incorrect next_major, next_minor and next_patch in prerelease SemverVersion #121

>>> from univers.versions import SemverVersion
>>> SemverVersion("1.0.0-beta").next_major()
SemverVersion(string='1.0.0')

>>> SemverVersion("1.0.0-beta").next_minor()
SemverVersion(string='1.0.0')

>>> SemverVersion("1.0.0-beta").next_patch()
SemverVersion(string='1.0.0')

>>> import semver
>>> version = semver.VersionInfo.parse("1.0.0-beta")
>>> str(version)
'1.0.0-beta'
>>> str(version.bump_major())
'2.0.0'
>>> str(version.bump_minor())
'1.1.0'
>>> str(version.bump_patch())
'1.0.1'