fix ebook-overfulls
Add .travis.yml for a Travis-CI instance

Someone with admin bits for this repo will need to go to
https://travis-ci.org/profile and sign in with github and enable Travis
for this repo.

If Travis is enabled, new commits and pull requests get auto-built by
Travis.  If someone is feeling ambitious, it's possible to make Travis
do other tasks, like auto-uploading a new version of the book somewhere,
or parsing `git blame` to update the eratta numbers and pushing the
appropriate eratta labels.
Merge pull request #540 from JasonGross/travis

Do you want a Travis-CI instance?
Merge pull request #537 from TimRichter/fixoverfulls

fix ebook-overfulls, close #491
close #487
unmarked errata for 8c6f3e8
mention potential pitfall proving 2.8.1, close #494
clarify uniqueness principle for truncations, close #512
Merge pull request #543 from mikeshulman/truncuniq

clarify uniqueness principle for truncations, close #512
Merge pull request #542 from mikeshulman/thm2.8.1

mention potential pitfall proving 2.8.1, close #494
Merge pull request #541 from mikeshulman/after-lemma-2.1.2

close #487
mark errata
errata-marking bash script