Replace -d by --debug in JRUBY_OPTS for travis

It seems that for coverage to work properly the "--debug" option is
needed when using JRuby.

Author: Yohan Robert

.travis.yml

@@ -24,7 +24,7 @@ script:
 
 env:
   global:
-    - "JRUBY_OPTS='--dev -J-Xmx1024M' -d"
+    - "JRUBY_OPTS='--dev -J-Xmx1024M --debug'"
   matrix:
     - "RAILS_VERSION=4.0"
     - "RAILS_VERSION=4.1"