Minor yum, permission and URL fixes

[j0yu]

• Dockerfile: Fixed yum clean all from being part of the yum install arguments
• build_docs.sh: Fixed permissions of generated docs from just root only
• travis-generate-docs.py: Fallback to DOC_* before using dummy URL.
• Version to release as? (need to edit changelog.md)

[j0yu]

Not sure if CI build fails because we're merging from a fork, rather than a branch within shotgunsoftware/tk-doc-generator. Does not seem to be picking up shotgunsoftware's AWS_S3_ACCESS_KEY

------------------------------------------------------
Build completed.
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INFO Exit code: 0
INFO Uploading build result to S3...
ERROR An exception was raised!
Traceback (most recent call last):
  File "./travis-generate-docs.py", line 225, in <module>
    main()
  File "./travis-generate-docs.py", line 177, in main
    aws_access_key_id=os.environ["AWS_S3_ACCESS_KEY"],
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/UserDict.py", line 40, in __getitem__
    raise KeyError(key)
KeyError: 'AWS_S3_ACCESS_KEY'
INFO Exiting with code 1.
The command "python ./travis-generate-docs.py" exited with 1.