travis: error on compiler warnings

One of our goals is to have our code free of any warnings. Due to the
recent switch to Ubuntu 14.04 on Travis, the last warning regarding some
preprocessor-magic in the curl-headers has been fixed and as such, the
goal of zero warnings is now reached for Travis CI. In order to avoid
introducing new warnings via pull requests, we can now enable building
with `-Werror` and turn compiler warnings into errors instead, causing
the CI jobs to fail.

This build does so by passing the newly introdcued `-DENABLE_WERROR`
flag to CMake for all Travis jobs.

.travis.yml

@@ -18,8 +18,8 @@ env:
   - secure: "YnhS+8n6B+uoyaYfaJ3Lei7cSJqHDPiKJCKFIF2c87YDfmCvAJke8QtE7IzjYDs7UFkTCM4ox+ph2bERUrxZbSCyEkHdjIZpKuMJfYWja/jgMqTMxdyOH9y8JLFbZsSXDIXDwqBlC6vVyl1fP90M35wuWcNTs6tctfVWVofEFbs="
   - GITTEST_INVASIVE_FS_SIZE=1
  matrix:
-  - OPTIONS="-DTHREADSAFE=ON -DCMAKE_BUILD_TYPE=Release"
-  - OPTIONS="-DTHREADSAFE=OFF -DBUILD_EXAMPLES=ON"
+  - OPTIONS="-DTHREADSAFE=ON -DCMAKE_BUILD_TYPE=Release -DENABLE_WERROR=ON"
+  - OPTIONS="-DTHREADSAFE=OFF -DBUILD_EXAMPLES=ON -DENABLE_WERROR=ON"
 
 dist: trusty
 sudo: true