Install gcc-multilib in travis

This fixes the following error
/usr/include/features.h:374:25: fatal error: sys/cdefs.h: No such file or directory

.travis.yml

@@ -3,7 +3,7 @@ compiler:
   - gcc
   - clang
 before_install:
-  - sudo apt-get install -y lcov libasan*
+  - sudo apt-get install -y gcc-multilib lcov libasan*
   - gem install coveralls-lcov
 script:
     - autoreconf -iv