Fix issue #1433: parse requirements in markers

* InstallRequirement supports PEP 426 markers
* RequirementSet.add_requirement() ignores an InstallRequirement if
  markers don't match.

fixes #1433
Correct - we removed the initial "1." so version 6.X is what was previously being referred to as 1.6.X. By the way, a minor nit,  "Since version 6.0" would be better grammatically.