Add frontend host_inject.rb, guarding against older versions

These versions may be unable to execute - or even parse - the main host
injection script.

lib-injection/host_inject.rb

@@ -0,0 +1,7 @@
+# This file's intent is to be parseable and executable by all ruby versions
+# to call into the main one only for versions for which it is known-compatible
+# with at the language level.
+
+if RUBY_VERSION >= '2.5.'
+  require File.expand_path(File.join(File.dirname(__FILE__), 'host_inject_main.rb'))
+end