Bug with LLVM build version is fixed.

Author: chrisseaton

doc/user/installing-llvm.md

@@ -29,9 +29,7 @@ yum install libcxx-devel
 
 ## Ubuntu
 
-The tested version of LLVM for Ubuntu is 3.8. At the moment this may be
-incompatible with GraalVM for installing some C extensions as bitcode in
-GraalVM binary releases is built with 4.0.
+The tested version of LLVM for Ubuntu is 3.8.
 
 For building C extensions you need to install:
 

test/truffle/docker/ubuntu/Dockerfile

@@ -93,6 +93,5 @@ RUN gem install oily_png
 RUN ruby -roily_png -e 'raise unless defined?(OilyPNG::Color)'
 
 # Check we can install and run a C++ extension gem
-# This is broken on Ubuntu because ruby.bc is built with LLVM 4.0
-#RUN gem install unf
-#RUN ruby -runf -e 'raise unless defined?(UNF)'
+RUN gem install unf
+RUN ruby -runf -e 'raise unless defined?(UNF)'