Use htmlproofer, not htmlproof, in travis script

Signed-off-by: Rob Hoes <[email protected]>

Author: robhoes

.travis-ci.sh

@@ -14,4 +14,4 @@ mv _site ${TEST_BASEURL}
 mkdir _site
 mv ${TEST_BASEURL} _site
 export NOKOGIRI_USE_SYSTEM_LIBRARIES=true  # speeds up install of html-proofer
-bundle exec htmlproof --check-html ./_site
+bundle exec htmlproofer --check-html ./_site