Add linting for yaml files (#186)

* rename check-scripts target to lint
* install yamllint through pip on travis
* add yamllint config, do not fail on too long lines
* fix yaml linting errors
* remove circular dependency

.yamllint

@@ -0,0 +1,7 @@
+extends: default
+
+rules:
+  # 80 chars should be enough, but don't fail if a line is longer
+  line-length:
+    max: 80
+    level: warning