Default to hermetic clang on Linux.

Change-Id: I25ed8ec182942d67ea61ff9b1d0628abbf465ea5
diff --git a/.travis.yml b/.travis.yml
index 481e2b1..8691c5f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -53,7 +53,7 @@
         apt:
           packages:
             - clang
-      env: CFG=linux_trusty-clang-x86_64-debug GN_ARGS="is_debug=true"
+      env: CFG=linux_trusty-clang-x86_64-debug GN_ARGS="is_debug=true is_hermetic_clang=false"
     - os: linux
       dist: trusty
       sudo: true