.travis.yml: Install webminspector

For test_that_video_index_is_written_on_eos (see commit b7a37493).
test_that_video_index_is_written_on_eos: Show webminspector output on failure

To debug failures on the travis-ci continuous integration service.
test_that_video_index_is_written_on_eos: Generate longer video file

When the test is run by the Travis continuous integration service, it
was intermittently failing with:

    Traceback (most recent call last):
      File "/home/vagrant/webminspector/webminspector.py", line 116, in <module>
        main()
      File "/home/vagrant/webminspector/webminspector.py", line 97, in main
        pos = segment.SearchLevelOneElements(file_, pos)
      File "/home/vagrant/webminspector/webm.py", line 560, in SearchLevelOneElements
        size, start_pos = ReadOneByteForElementIdSize(file_, pos)
      File "/home/vagrant/webminspector/webm.py", line 53, in ReadOneByteForElementIdSize
        file_.seek(pos)
    TypeError: 'NoneType' object cannot be interpreted as an index
    Didn't find 'Cue Point' in /tmp/stb-tester.zna/webminspector.log

The same error happened when running the test in the Ubuntu VM under
extra/vm, even though the generated video played correctly in Firefox
(and was correctly seekable). Presumably webminspector.py doesn't handle
short videos correctly. After increasing the video's length to 5 seconds
the test now passes reliably.