travis: Use unified diffs for failed tests.

.travis.yml

@@ -20,4 +20,4 @@ script:
   - (cd tests && MICROPY_CPYTHON3=python3.3 ./run-tests)
 
 after_failure:
-  - (cd tests && for exp in *.exp; do testbase=$(basename $exp .exp); echo -e "\nFAILURE $testbase"; diff $testbase.exp $testbase.out; done)
+  - (cd tests && for exp in *.exp; do testbase=$(basename $exp .exp); echo -e "\nFAILURE $testbase"; diff -u $testbase.exp $testbase.out; done)