mkdir does not throw errors

.travis.yml

@@ -26,7 +26,7 @@ before_install:
    - sudo pip install cpp-coveralls
    - export CC=/usr/bin/gcc-4.8
 script: 
-   - mkdir build
+   - mkdir -p build
    - cd build
    - cmake -DGENERATE_DOCUMENTATION=ON .. 
    - echo "Production build"