Use new notebook exclusion in treon==0.1.3

This means that we can throw away the workaround we had in `.travis.yml`.
Not a super big change, but it feels nice.

Author: natanlao

.travis.yml

@@ -13,14 +13,9 @@ install:
 - pip install -r test/requirements-dev.txt
 - find . -name "requirements.txt" -exec pip install -r "{}" \;
 
-before_script:
-# To ignore a directory, we can just delete the directory
-# so that treon doesn't see it.
-- xargs -t rm -rf < test/ignore
-
 script:
 # Test that notebooks run successfully
-- treon
+- eval "treon $(awk '{printf " --exclude " $0}' test/ignore)"
 # Naive test to check that output committed with notebook-
 - find . -name "*.ipynb" \
     -exec test $(grep -oc '"outputs": ' "{}") -eq $(grep -oc '"cell_type": "code"' "{}") || exit 1 \;