Fix Clang APT sources for Travis CI.

Change-Id: Ib9288292d63999afcee273475665427b334be787
Reviewed-on: https://code-review.googlesource.com/14473
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/.travis.yml b/.travis.yml
index 6645ef6..8c9acca 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -58,7 +58,7 @@
         apt:
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-trusty-3.5
+            - llvm-toolchain-precise-3.5
           packages:
             - clang-3.5
       env:
@@ -68,7 +68,7 @@
         apt:
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-trusty-3.6
+            - llvm-toolchain-precise-3.6
           packages:
             - clang-3.6
       env:
@@ -78,7 +78,7 @@
         apt:
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-trusty-3.7
+            - llvm-toolchain-precise-3.7
           packages:
             - clang-3.7
       env:
@@ -88,7 +88,7 @@
         apt:
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-trusty-3.8
+            - llvm-toolchain-precise-3.8
           packages:
             - clang-3.8
       env:
@@ -97,7 +97,8 @@
       addons:
         apt:
           sources:
-            - llvm-toolchain-trusty-3.9
+            - ubuntu-toolchain-r-test
+            - llvm-toolchain-precise-3.9
           packages:
             - clang-3.9
       env: