.travis.yml: Enable C99 mode for older gcc versions

Currently Travis builds fail on the edit_distance module, because that
module uses C99 extensions, which aren't enabled by default on the
Ubuntu Precise compiler version.  Force it to allow these, by adding the
-std=gnu99 option to the compiler.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

.travis.yml

@@ -5,6 +5,9 @@ compiler:
   - gcc
   - clang
 
+env:
+    - CFLAGS="-std=gnu99"
+
 script:
     - make -j2 -k quiet=1
     - make -k check quiet=1