Use gcc-6 providing C11 features

.travis.yml

@@ -1,8 +1,20 @@
 language: cpp
+matrix:
+  include:
+  # works on Precise and Trusty
+  - os: linux
+    addons:
+      apt:
+        sources:
+          - ubuntu-toolchain-r-test
+        packages:
+          - g++-6
+    env:
+      - MATRIX_EVAL="CC=gcc-6 && CXX=g++-6"
 compiler:
   - gcc
-  - clang
 before_install:
+  - eval "${MATRIX_EVAL}"
   - ci/make-tmp.sh
   - ci/install-fesvr.sh
   - ci/install-verilator.sh