Update ruby-build -> 20181207

changes from previous version since 20181019:
- 20181106
  - Add 2.6.0-preview3
  - truffleruby-1.0.0-rc8, 1.0.0-rc9
- 20181207
  - Add 2.6.0-rc1
  - Add jruby-9.2.1.0, 9.2.2.0, 9.2.3.0, 9.2.4.0, 9.2.4.1, 9.2.5.0
  - Add truffleruby-1.0.0-rc10

lib/utils.sh

-RUBY_BUILD_VERSION="20181019"
+RUBY_BUILD_VERSION="20181207"
 RUBY_BUILD_TAG="v$RUBY_BUILD_VERSION"
 
 echoerr() {