[semver] namespace semver to avoid require collisions

If Puppet and r10k are required in the same namespace then the vendored
copy of SemVer can act poorly when interacting with Puppet's copy. This
commit moves the SemVer class to r10k/semver and namespaces the class
under the R10K namespace.
Merge branch 'defect/master/rename-semver'
Update CHANGELOG and version for 1.1.2