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
diff --git a/.travis.yml b/.travis.yml
index 4d29c5e..2b57d8d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,7 +3,7 @@
   - 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