Revert "travis: use more recent libstdc++"

This reverts commit 075bf331ac.
This is no longer needed since the llvmlite-artiq package for
linux-64 has been rebuilt on an older Ubuntu (14.04 LTS)

.travis.yml

@@ -2,12 +2,6 @@ language: python
 python:
   - '3.4'
 sudo: false
-addons:
-  apt:
-    sources:
-      - ubuntu-toolchain-r-test
-    packages:
-      - libstdc++6
 env:
   global:
     - BUILD_SOC=1