Check for external resources in CI

Fixes xsf#212

.travis.yml

@@ -8,5 +8,7 @@ before_install:
   - chmod +x git-ratchet
   - git fetch https://github.com/xsf/xeps.git refs/notes/*:refs/notes/*
 script:
+  - xmllint --nonet --noout --noent --loaddtd --xpath "//img/@src[not(starts-with(., 'data:'))]" *.xml 2>/dev/null
+  - test $(xmllint --nonet --noout --noent --loaddtd --xpath "//img/@src[not(starts-with(., 'data:'))]" *.xml 2>/dev/null | wc -c) == 0
   - make all
   - echo "lint,$(xmllint --nonet --noout --noent --loaddtd --valid *.xml 2>&1 | wc -l)" | ./git-ratchet check -v