Release 0.18.1 (2018-10-31)

Baseline: c062b1f1

Cherry picks:

   + 2834613f:
     Include also ext jars in the bootclasspath jar.
   + 2579b791:
     Fix toolchain_java9 on --host_javabase=<jdk9> after
     7eb9ea15
   + faaff7fa:
     Release notes: fix markdown
   + b073a18e:
     Fix NestHost length computation Fixes #5987
   + bf6a63d6:
     Fixes #6219. Don't rethrow any remote cache failures on either
     download or upload, only warn. Added more tests.
   + c1a7b4c5:
     Fix broken IdlClassTest on Bazel's CI.
   + 71926bc2:
     Fix the Xcode version detection which got broken by the upgrade
     to Xcode 10.0.
   + 86a8217d:
     Temporarily restore processing of workspace-wide tools/bazel.rc
     file.
   + 914b4ce1:
     Windows: Fix Precondition check for addDynamicInputLinkOptions
   + e0257260:
     Update turbine

Important changes:

  - Fix regression #6219, remote cache failures