[Fix #581] Fix font locking for keywords starting with a number

[OknoLombarda]

Fixes https://github.com/clojure-emacs/clojure-mode/issues/581

Context

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Before submitting a PR mark the checkboxes for the items you've done (if you think a checkbox does not apply, then leave it unchecked):

• [x] The commits are consistent with our contribution guidelines.
• [x] You've added tests (if possible) to cover your change(s). Bugfix, indentation, and font-lock tests are extremely important!
• [x] You've run M-x checkdoc and fixed any warnings in the code you've written.
• [x] You've updated the changelog (if adding/changing user-visible functionality).
• [ ] You've updated the readme (if adding/changing user-visible functionality).

Thanks!