travis CHANGE disable valgrind tests on OS X

On OSX, valgrind prints erros from the system libraries, so it is useless
to check memory issues in libyang itself.
diff --git a/.travis.yml b/.travis.yml
index 29c8b72..45b2768 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -32,7 +32,7 @@
 
 script:
   - mkdir build && cd build
-  - if [ "$TRAVIS_OS_NAME" = "osx" ]; then cmake ..; fi
+  - if [ "$TRAVIS_OS_NAME" = "osx" ]; then cmake -DENABLE_VALGRIND_TESTS=OFF ..; fi
   - if [ "$TRAVIS_OS_NAME" = "linux" ]; then cmake ..; fi
   - make -j2 && ctest --output-on-failure
   - cd -