Do debug build in Travis

The debug builds do extra check, so chose that mode for Travis so we
get early warnings for problems.
diff --git a/.travis.yml b/.travis.yml
index 74878fc..ceafcbb 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -16,5 +16,5 @@
  - popd
 
 script:
- - cmake . && make
+ - cmake -DCMAKE_BUILD_TYPE=Debug . && make
  - cd java && cmake . && make