Update guide on CONTRIBUTING

As a contributor you're no longer required to add NEWS items. We'll do
that ourselves :)
Remove require that was causing warnings
Tweak allow_value failure_message

This commit changes the failure message that `allow_value` generates so
that it reads a bit nicer.

When a call to #valid? resulted in validation messages, `allow_value`
formerly failed with this message:

    Expected errors to include "the message" when attr is set to "some value", got errors: ["another message (attribute: \"attr\", value: \"some value\")", "some other message (attribute: \"attr2\", value: \"some other value\")"]

Now it fails with this message:

    Expected errors to include "the message" when attr is set to "some value",
    got errors:
    * "another message" (attribute: attr, value: "some value")
    * "some other message" (attribute: attr2, value: "some other value")

Similarly, when a call to #valid resulted in an exception, `allow_value`
formerly failed with this message:

    Expected errors to include "the message" when attr is set to "some value", got: some message

Now it fails with this message:

    Expected errors to include "the message" when attr is set to "some value",
    got: "some message"