Add "backports" gem as a dependency.

This is not required for bud itself, but some of the examples (e.g., chat) make
use of it; it is a relatively harmless dependency, so just add it to simply the
first-time UX.
Add .travis.yml file for Travis CI.
Minor code cleanup.
Merge pull request #300 from JoshRosen/master

Add Travis CI configuration
Update release notes for PR #300.
Merge pull request #299 from ilikerps/master

Issue #289. Disallow <= outside bloom blocks.
Update release notes.

Per 331833a3f974021780d6cc39d4ed8f75c7e2ffd4; closes #289.
Add test case for stdio.

Check that reading from / writing to pipe IO objects works. This functionality
is being used by the UCLA folks, so it seems prudent to verify that it works.
Fix bug in join rules that reference nested collections.

Closes #301.
Optimize notin self joins.

We previously sent a duplicate copy of every tuple; we no longer do this.
resolve issue #304
Tweak test case.

Also update release notes for #304 fix.
Rename lset#apply_monotone to lset#apply.

Also, allow #apply to take both monotone functions or morphisms, rather than
just monotone functions. The idea is that most users can just invoke apply;
whereas if the user wants better performance and understands the monotone
function vs. morphism distinction, they can use apply_morph when appropriate.

It would be possible to remove #apply_morph and have #apply choose at runtime
between acting like a morphism or a monotone function, depending on the nature
of its first argument. This would be pretty easy to implement but would require
special-casing support for #apply into the lattice-core infrastructure, which
would be a hacky layering violation. So I haven't done that for now.
Implement lmap#filter.

This takes an lmap whose values are elements of the lbool lattice and returns an
lmap containing only those k/v pairs where the value is true.
Test case for lattice-to-set conversion behavior.
Support deletion rules with lattice values on RHS.
Minor code cleanup.

`to_delete` is not defined in the BudCollection parent class, and probably
needn't be public knowledge anyway.
Tweak BUD_DEBUG output.
Code cleanup.

Clarify comments, improve error messages.
Add an important clarifying comment.