Don't run souffle tests in Travis.

The test no longer works on MacOS, as subversion installation fails in
Travis.  Since these tests run in gitlab anyway, there is no hard in
disabling them.

.travis.yml

@@ -43,7 +43,7 @@ cache:
 
 env:
    - TOOL=stack TEST_SUITE='-p tutorial' BUILD_BINARY=1
-   - TOOL=run_souffle_tests
+   # - TOOL=run_souffle_tests
    # - TOOL=stack TEST_SUITE='-p redist' JAVA=1
    # - TOOL=stack TEST_SUITE='-p span_uuid'
    # - TOOL=stack TEST_SUITE='-p souffle0'