Install importlib for travis tests on python 2.6

Signed-off-by: Martin Raspaud <martin.raspaud@smhi.se>

.travis.yml

@@ -14,6 +14,7 @@ matrix:
 install:
 - pip install .
 - if [[ $TRAVIS_PYTHON_VERSION == "2.6" ]]; then pip install unittest2; fi
+- if [[ $TRAVIS_PYTHON_VERSION == "2.6" ]]; then pip install importlib; fi
 - pip install h5py
 - pip install coveralls
 - pip install pyorbital