test: Add dateutil as a Travis-CI dependency

The package python-dateutil is used for the management of timestamps in
the cache of LilyPond versions

Author: Cecca

.travis.yml

@@ -20,6 +20,7 @@ cache:
 # Download and install LilyPond (stable and devel)
 # when not present already (cached between builds)
 install:
+  - pip install python-dateutil --user
   - python ./test/install_lilypond.py
 
 # This is the script that will actually run the tests