[travis] Run latexmk silently. Vastly reduces log output.

Author: tkoeppe

.travis.yml

@@ -14,7 +14,7 @@ script:
   # Build std.pdf
   - pushd source
   - if [ "$BUILD_TYPE" = "latexmk" ]; then
-      latexmk -pdf std;
+      latexmk -pdf std -silent;
       ! grep -Fe "Overfull \\hbox" std.log;
     fi
   - if [ "$BUILD_TYPE" = "make" ]; then