Update TruffleRuby version in CI

Author: pitr-ch

.travis.yml

@@ -15,7 +15,7 @@ matrix:
     - name: TruffleRuby Latest
       rvm: system
       install:
-        - export TRUFFLERUBY_VERSION=1.0.0-rc7
+        - export TRUFFLERUBY_VERSION=1.0.0-rc8
         - curl -L https://github.com/oracle/truffleruby/releases/download/vm-$TRUFFLERUBY_VERSION/truffleruby-$TRUFFLERUBY_VERSION-linux-amd64.tar.gz | tar xz
         - export PATH="$PWD/truffleruby-$TRUFFLERUBY_VERSION-linux-amd64/bin:$PATH"
         - gem install bundler