selftests: Skip `test_clock_visualisation` on Travis
selftests: Skip `test_clock_visualisation` on Travis

Travis has tesseract 3.02; the `tesseract_user_patterns` parameter to
`stbt.ocr`, used in this test, requires tesseract 3.03.

If I don't use `tesseract_user_patterns`, the test is more flaky (see
https://github.com/drothlis/stb-tester/pull/261 ).

LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester