Revert "Install LLVM 3.8 explicitly in .travis.yml"

This reverts commit 5fd294d.  It looks like
Travis CI does not yet support explicit installation of LLVM 3.8.

Author: spakin

.travis.yml

@@ -10,7 +10,7 @@ sudo: false
 addons:
   apt:
     sources:
-      - llvm-toolchain-precise-3.8
+      - llvm-toolchain-precise
       - ubuntu-toolchain-r-test
     packages:
       - ccache