Add testing to ensure the book "builds" at all times

[mbrukman]

The book is built using the script book/make-book.sh; however, syntax errors in the AsciiDoc file can break that build.

We should add a CI build (e.g., Travis) to test every PR and the master branch to ensure that it continues to build.

Proposal:

• @krlawrence needs to visit https://travis-ci.org/krlawrence/graph and enable the repo
• be sure to set the option in settings "only run when .travis.yml is present
• @mbrukman offers to add a .travis.yml to run the build steps in a PR, which will also test that config

[krlawrence]

I like your proposal, this has been on my todo list. All the time it has been just me doing the writing it has not seemed like a big deal as I always do the build myself but I agree if others start to contribute text this will be very valuable.

[krlawrence]

Work is just getting started on a Second Edition. We can use this as a good excuse to revisit all kinds of automation possibilities.