Install llvm instead of clang-3.7

It seems like the package no longer exists under that name
in Bionic so we'll have to do transition to this package
instead.

zuul.d/playbooks/pre.yaml

@@ -8,7 +8,7 @@
       apt:
         name:
           - lcov
-          - clang-3.7
+          - llvm
           - libev-dev
           - libc-ares-dev
           - libstdc++-4.8-dev