Add Travis CI and update tests. Start a Changelog.
* .travis.yml: new file
* README.mdown: Add Travis CI badge.
* Rakefile: use ert-run-tests-batch-and-exit.
* yasnippet-tests.el (example-for-issue-404-external-emacs): Use
`yas-with-snippet-dirs' to not depend on bundled snippets.
(yas--call-with-temporary-redefinitions): Use `cl-labels' since no
cl-flet in emacs-24.3's cl-lib.el.
(loading-with-cyclic-parenthood): prog-mode doesn't exist in emacs
24.3.
(yas-batch-run-tests): remove it.
* yasnippet.el (require): require cl-lib during byte-compilation
and load.