Travis: build with -Werror

Rationale:

new code/pull requests should not produce any warnings as per
docs/Readme.developers.txt. This commit encodes this requirement
into CI.

We could add this to the default CFLAGS in configure, but this
might be annoying during development where some warnings may be
allowed.

Author: mhaas

.travis.yml

@@ -11,8 +11,11 @@ install:
 script:
   - ./.travis_configure_wrapper.sh && make
 env:
-  - BUILD_TYPE=normal
-  - CYASSL="3.3.2" BUILD_TYPE=cyassl
+  global:
+    - CFLAGS="-Werror"
+  matrix: 
+    - BUILD_TYPE=normal
+    - CYASSL="3.3.2" BUILD_TYPE=cyassl
 cache:
   directories:
     - dependencies-src