Travis: Fail build on warnings.

This may be a bit strict, but let us try. We don't want to miss warnings
that may have security implications.

.travis.yml

@@ -5,4 +5,4 @@ env:
 install:
   - sudo apt-get update -qq
   - sudo apt-get install -qq "$APACHE_PKG" libglib2.0-dev liblasso3-dev libssl-dev
-script: ./autogen.sh && ./configure && make && make distfile
+script: ./autogen.sh && ./configure CFLAGS=-Werror && make && make distfile