travis: also install python3-coverage system package w/ Python 3.2

See also:
https://bitbucket.org/ned/coveragepy/issues/407/coverage-failing-on-python-325-using#comment-21934227

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>

.travis.yml

@@ -28,7 +28,7 @@ before_install:
           || sudo apt-get install -qq $(python -c
              'from sys import exit, version_info as v; exit(v[1] == 2)'
              && echo "python-lxml python-setuptools"
-             || echo "python3-lxml python3-setuptools")
+             || echo "python3-lxml python3-setuptools python3-coverage")
 
 install:
         - pushd __root__