travis: install llvm-7

Author: minad

.travis.yml

@@ -166,6 +166,10 @@ matrix:
     # Link time optimization
     - env: SANITIZER=1 COMPILE_LTO=1 BUILDOPTIONS='--with-cc=gcc-5 --with-m64 --with-travis-valgrind'
     - env: SANITIZER=1 COMPILE_LTO=1 BUILDOPTIONS='--with-cc=clang-7 --with-m64 --with-travis-valgrind'
+      addons:
+        apt:
+          packages:
+            - llvm-7
 
     # GCC for the x86-64 architecture with restricted limb sizes
     # formerly started with the option "--with-low-mp" to testme.sh