Try to fix Travis CI test failure*

From https://travis-ci.org/xolox/python-deb-pkg-tools/builds/345842127:

dpkg-shlibdeps: error: no dependency information found
for /opt/python/2.7.14/lib/libpython2.7.so.1.0 (used
by /home/travis/virtualenv/python2.7.14/bin/python)

deb_pkg_tools/tests.py

@@ -434,7 +434,7 @@ def test_find_object_files(self):
 
     def test_find_system_dependencies(self):
         """Test the :func:`deb_pkg_tools.package.find_system_dependencies()` function."""
-        dependencies = find_system_dependencies([sys.executable])
+        dependencies = find_system_dependencies(['/usr/bin/python'])
         assert len(dependencies) >= 1
         assert any(re.match(r'^libc\d+\b', d) for d in dependencies)