configury: strip trailing whitespace from generated .travis.yml.

When a slingshot client does NOT use ldoc or luadoc, then
GENDOC is empty, and correctly commented out in the generated
.travis.yml, but leaves behind a trailing space after configure
substitutions that has to be removed manually.
* m4/slingshot.m4 (AC_CONFIG_FILES): Strip trailing whitespace.
* .travis.yml: Regenerate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>