Require xml2rfc < 2.10 in travis build

From version 2.10, option --basename changed its semantics and is now
expected to be a directory, which does not fit our current usage.

Alternatively, we could drop this option altogether (and thus avoid
version pinning) since I'm not sure what this is for.

Author: dlax

.travis.yml

@@ -1,6 +1,6 @@
 language: python
 install:
-- pip install xml2rfc
+- pip install "xml2rfc<2.10"
 before_script:
 - TAG=$(git tag -l --points-at HEAD)
 script: