Sync dist_utils.py with latest from st2 repo

[nmaludy]

Sync dist_utils.py with latest from st2 repo

Fix is related to: StackStorm/st2#4929

Fixes error in CI: https://travis-ci.org/github/StackStorm/st2/jobs/681137227

    ERROR: Command errored out with exit status 1:
     command: /home/travis/build/StackStorm/st2/virtualenv/bin/python2.7 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-BVwhVy/st2-auth-backend-flat-file/setup.py'"'"'; __file__='"'"'/tmp/pip-install-BVwhVy/st2-auth-backend-flat-file/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-mhz4SU
         cwd: /tmp/pip-install-BVwhVy/st2-auth-backend-flat-file/
    Complete output (7 lines):
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-install-BVwhVy/st2-auth-backend-flat-file/setup.py", line 32, in <module>
        install_reqs, dep_links = fetch_requirements(REQUIREMENTS_FILE)
      File "dist_utils.py", line 74, in fetch_requirements
        if req.link:
    AttributeError: 'ParsedRequirement' object has no attribute 'link'
    ----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

[arm4b]

Thanks! Merged it straight away to get the fix out in master.