Show test output on CI in Travis (#777)

* Showing test output only on failure

* Test commit to see the failed test

* Now making sure that the tests pass with no output

.travis.yml

@@ -14,6 +14,8 @@ before_install:
         sudo apt-get install -qq g++-4.7 && export CXX="g++-4.7" CC="gcc-4.7"
       fi
     fi
+env:
+  - CTEST_OUTPUT_ON_FAILURE=1
 before_script:
   - mkdir build
   - cd build