Canonicalize 'nil to nil when displaying default optional and
keyword arguments via slime-autodoc. On SBCL this turns
  (error 'simple-condition
         &rest arguments
         &key (format-arguments 'nil) (format-control 'nil))
into
  (error 'simple-condition
         &rest arguments
         &key format-arguments format-control)
which should be more readable.

* swank-arglists.lisp (canonicalize-default-arg): New function for
canonicalizing optional and keyword arguments.
(make-optional-arg, make-keyword-arg): Use it.
Test with 64-bit SBCL and clean up .travis.yml a bit

(We were previously using the 32-bit version because that version
 is older than the 64-bit version which was failing some autodoc
 tests.)