travis: add link to synergine

.travis.yml

   - rm modules/*
   - ln -s ../synergine/modules/xyzworld modules/xyzworld
   - ln -s ../synergine/modules/xzworld modules/xyworld
+  - ln -s ../synergine/synergine modules/synergine
 script:
   - coverage run tests.py --rcfile=.coveragerc -v && coverage report -m && python3.4 tests.py
 after_success: