travis.yml: for Python 2.6, test against html5lib 0.999999999.

html5lib 0.999999999 is the last version that supported Python 2.6.

html5lib/html5lib-python#330
html5lib/html5lib-python@5f637af

Author: jwilk

.travis.yml

@@ -26,7 +26,8 @@ install:
 - if [ "$TRAVIS_PYTHON_VERSION" = '2.6' ]; then PYICU_VERSION=1.9.3; fi
 - pip install --verbose pyicu${PYICU_VERSION:+==$PYICU_VERSION}
 - pip install --upgrade setuptools
-- pip install html5lib
+- if [ "$TRAVIS_PYTHON_VERSION" = '2.6' ]; then HTML5LIB_VERSION=0.999999999; fi
+- pip install html5lib${HTML5LIB_VERSION:+==$HTML5LIB_VERSION}
 - if [ "$TRAVIS_PYTHON_VERSION" = '2.6' ]; then PILLOW_VERSION=3.4.2; fi
 - pip install --verbose pillow${PILLOW_VERSION:+==$PILLOW_VERSION}
 - pip install pydiatra pyflakes