Problem: ASAN test doesn't print good traces with GCC 4.8

Solution: add Ubuntu toolchain PPA and use GCC 6

.travis.yml

@@ -57,6 +57,14 @@ matrix:
     os: osx
   - env: BUILD_TYPE=default CURVE=tweetnacl DRAFT=enabled ADDRESS_SANITIZER=enabled
     os: linux
+    dist: trusty
+    addons:
+      apt:
+        sources:
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
+        packages:
+        - g++-6
+        - gcc-6
 
 sudo: required
 

ci_build.sh

@@ -17,6 +17,11 @@ if [ $BUILD_TYPE == "default" ]; then
 
     if [ -n "$ADDRESS_SANITIZER" ] && [ "$ADDRESS_SANITIZER" == "enabled" ]; then
         CONFIG_OPTS+=("--enable-address-sanitizer=yes")
+        CONFIG_OPTS+=("CXX=g++-6")
+        CONFIG_OPTS+=("CC=gcc-6")
+        # workaround for linker problem with ASAN options in GCC
+        # http://stackoverflow.com/questions/37603238/fsanitize-not-using-gold-linker-in-gcc-6-1
+        CONFIG_OPTS+=("LDFLAGS=-fuse-ld=gold")
     fi
 
     if [ -z $CURVE ]; then