Upgrade ZXing jars to 3.1.1

Steps:
- git clone github.com/zxing/zxing
- mvn package
- copy over the /core and /javase jars
Add Travis CI configuration.
Remove sdoc development dependency.

Sadly, this adds a dependency on the json gem, which blows up on the
latest Ruby 2.2 HEAD.

Comfortable re-adding it if Ruby 2.2 HEAD stops exploding, but I don't
believe we're generating docs anyway.
Bump version to 0.4.0
Merge pull request #10 from ecin/ecin/update-zxing

Update zxing jars.