TravisCI: fix issue with single error being ignored.

When codestyle check produces single error message, `$num_errors`
remains zero due to lack of newline symbol in the end of error message.
`echo -n` was used to get zero value for empty line, but unfortunately
it affects non-empty line as well.

Now we simply check if there anything in the `$errors` variable.

.travis.yml

@@ -11,9 +11,8 @@ jobs:
 
       script:
         - errors=`find . -name "*.py" -exec pycodestyle --ignore W503,W292 {} \; `
-        - num_errors=`echo -n "$errors" | wc -l`
         - |
-            if [ $num_errors -eq 0 ]; then
+            if [ -z "$errors" ]; then
               true
             else
               echo "$errors"