switch one build to Release mode

This turns on optimization which allows the compiler to discover more
problems and omit some more warnings.

.travis.yml

@@ -37,7 +37,7 @@ script: ./travis.sh
 env:
   matrix:
     - SHARED_LIB=OFF STATIC_LIB=ON CMAKE_PKG=OFF BUILD_TYPE=Debug VERBOSE=1
-    - SHARED_LIB=OFF STATIC_LIB=ON CMAKE_PKG=OFF BUILD_TYPE=Debug VERBOSE=1 CXX_FLAGS=-std=c++11
+    - SHARED_LIB=OFF STATIC_LIB=ON CMAKE_PKG=OFF BUILD_TYPE=Release VERBOSE=1 CXX_FLAGS=-std=c++11
 notifications:
   email: false
 sudo: false