Update .travis.yml

Add diffing of failure logs

.travis.yml

@@ -10,3 +10,6 @@ before_script:
 script:
   - make -C unix
   - cd tests && MICROPY_CPYTHON3=python3.3 ./run-tests
+
+after_failure:
+  - for exp in *.exp; do testbase=$(basename $exp .exp); echo -e "\nFAILURE: $testbase"; diff $testbase.exp $testbase.out; done