travis: use default system clang for sanitizers

.travis.yml

@@ -45,15 +45,9 @@ matrix:
           - CXXFLAGS="-g -O2 -fno-omit-frame-pointer -fsanitize=undefined -fsanitize=address"
           - ASAN_OPTIONS=check_initialization_order=1:detect_stack_use_after_return=1:detect_leaks=1
           - UBSAN_OPTIONS=print_stacktrace=1:report_error_type=1
-      before_install: export CXX=clang++-3.8
       addons:
         apt:
-          sources:
-            - llvm-toolchain-precise-3.8
-            - ubuntu-toolchain-r-test
           packages:
-            - clang-3.8
-            - llvm-3.8 # for llvm-symbolizer
             - expect
             - gettext
             - libncurses5-dev