make install on travis with Ubuntu 14.04 broken

@rene.milk wrote:

I'm moving to 14.04 on Travis now. I'm using the release 2.4 branches, but I'm getting an error where I'm pretty sure I saw a related commit messages some time in the past:

Building a virtual env in /home/travis/dune_build/dune-testtools/python2-env...
-- Writing a wrapper script around virtualenv to /home/travis/dune_build/dune-testtools/dune-env-2
-- Writing a wrapper script around virtualenv to /home/travis/dune_build/dune-testtools/dune-env
Building a virtual env in /home/travis/dune_build/dune-testtools/python3-env...
Falling back to terrible things to workaround Ubuntu bugs...
-- Writing a wrapper script around virtualenv to /home/travis/dune_build/dune-testtools/dune-env-3
-- Writing a wrapper script around virtualenv to /home/travis/dune_build/dune-testtools/dune-env
Traceback (most recent call last):
  File "/home/travis/dune_build/dune-testtools/python3-env/bin/dune_extract_static.py", line 12, in <module>
    exec(compile(open(__file__).read(), __file__, 'exec'))
  File "/home/travis/src/dune-testtools/python/scripts/dune_extract_static.py", line 4, in <module>
    from dune.testtools.static_metaini import extract_static_info
  File "/home/travis/src/dune-testtools/python/dune/testtools/static_metaini.py", line 2, in <module>
    from dune.testtools.parser import CommandToApply
  File "/home/travis/src/dune-testtools/python/dune/testtools/parser.py", line 8, in <module>
    from dune.common.parametertree.dotdict import DotDict
ImportError: No module named 'dune.common'
Traceback (most recent call last):
  File "/home/travis/dune_build/dune-testtools/python3-env/bin/dune_expand_metaini.py", line 12, in <module>
    exec(compile(open(__file__).read(), __file__, 'exec'))
  File "/home/travis/src/dune-testtools/python/scripts/dune_expand_metaini.py", line 3, in <module>
    from dune.testtools.metaini import expand_meta_ini, write_configuration_to_ini
  File "/home/travis/src/dune-testtools/python/dune/testtools/metaini.py", line 31, in <module>
    from dune.common.parametertree.dotdict import DotDict
ImportError: No module named 'dune.common'
CMake Error at cmake/modules/DuneCMakeAssertion.cmake:52 (message):
  Error in discard test 1
Call Stack (most recent call first):
  cmake/modules/DuneCMakeAssertion.cmake:75 (fail_assert)
  dune/testtools/test/cmakediscard/CMakeLists.txt:10 (dune_assert)

The bug only appears when installing dune-python.