Travis: disable Android, enable extra OSX

[dhardy]

• Disable Android test (see Travis failure on Android #701)
• Enable extra OSX tests ('os' key doesn't allow list)
• Shorten descriptions

[dhardy]

Build failure; see: rust-lang/libc#1232 (comment)

[dhardy]

I'd really like to merge this, but the doc-upload fails (also on restart). @huonw I believe this is your script; any idea?

$ travis-cargo --only nightly doc-upload
Traceback (most recent call last):
  File "/home/travis/.local/bin/travis-cargo", line 11, in <module>
    sys.exit(main())
  File "/home/travis/.local/lib/python2.7/site-packages/travis_cargo.py", line 433, in main
    manifest = Manifest(os.getcwd(), version)
  File "/home/travis/.local/lib/python2.7/site-packages/travis_cargo.py", line 70, in __init__
    self.manifest = json.loads(output.decode('utf-8'))
  File "/usr/lib/python2.7/json/__init__.py", line 338, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python2.7/json/decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python2.7/json/decoder.py", line 384, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded

[dhardy]

Rebased, without disabling Android test (which seems to work now).

[dhardy]

Finally, the test passes (??)