Raise error on Ruby 2.6 until it is supported

As the project does not tell the user that Ruby 2.6 is not yet
suypported, they frequently ask why their code is not working with the
following error:

    Error parsing classes in /root/manageiq/app/models/account.rb:
    RuntimeError: type should be a Symbol, not: 6

This commit is to prevent those questions until we can get Ruby 2.6
working.

Gemfile

@@ -1,4 +1,5 @@
-raise "Ruby versions less than 2.3.1 are unsupported!" if RUBY_VERSION < "2.3.1"
+raise "Ruby versions < 2.3.1 are unsupported!" if RUBY_VERSION < "2.3.1"
+raise "Ruby versions >= 2.6 are unsupported!" if RUBY_VERSION >= "2.6.0"
 
 source 'https://rubygems.org'