chore(.travis.yml): don't test packages when building nightly

.travis.yml

@@ -132,7 +132,8 @@ script:
   - make -j2
   - if [[ $TEST != OFF ]]; then yes "A" | travis_wait 60 ctest -j2 --output-on-failure; fi
   - |
-    if [[ $TEST_LEANPKG_REGISTRY == ON ]]; then
+    # don't test packages when building nightly
+    if [[ $TEST_LEANPKG_REGISTRY == ON && -z $LEAN_VERSION_STRING ]]; then
       (cd ../packages
        if [[ $TRAVIS_OS_NAME == linux ]]; then ../bin/leanpkg add "https://github.com/leanprover/smt2_interface"; fi
        ../bin/leanpkg configure