Use cider--font-lock-ensure for compatibility with Emacs 24.5

[xiongtx]

Fixes #2056.

[bbatsov]

This should be in the changelog as it's a bugfix for a stable release.

We should probably update travis to run the byte-compilation checks for all supported Emacs version (https://github.com/clojure-emacs/cider/blob/master/.travis.yml#L8) to avoid such issues down the road.

[xiongtx]

Added changelog entry and full test matrix for Emacs versions and tests.

At some point we should probably drop support for Emacs 24.4 and just go w/ 24.5.

I'm not familiar enough w/ Travis to know whether this line under matrix is necessary.

[xiongtx]

Looks like filling out the test matrix is already paying off--just discovered we're using directory-files-recursively, which is introduced by Emacs 25, without defining it in cider-compat.

[bbatsov]

I see that the tests are still failing because of directory-files-recursively. I'm also not sure is the checkdoc task working properly. I remember a few PRs with checkdoc errors that weren't flagged for some reason. Also - checkdoc hasn't changed much in a long time, do we need to run it on every supported Emacs?

[xiongtx]

I'm mystified as to why directory-files-recursively is causing the test to fail. I've posted a question on Emacs StackExchange.

I'm also not sure is the checkdoc task working properly. I remember a few PRs with checkdoc errors that weren't flagged for some reason.

checkdoc produces warnings, not errors, so the tests still pass.

There's no option to make checkdoc use error instead of warn, but we can run checkdoc-file on all source files and check whether the *Warnings* buffer exists after that. If it does, we can exit w/ an error.

For some reason checkdoc is not reporting warnings for Emacs 24.x, even though I've copied in check-doc error from Emacs 25 into cider-checks.el. Will look into this further when I have time.

checkdoc hasn't changed much in a long time, do we need to run it on every supported Emacs?

I don't see the harm. Travis CI has a 200 build limit IIRC and we're not anywhere near that.

[xiongtx]

I'm going to open another PR w/ the TravisCI stuff. We'll keep this PR focused on font lock.