Use verbose test output to help find segfaults

While testing null-byte support, the Ruby interpreter segfaulted but the
use of RSpec's default "dots" formatter made it difficult to see which
test had caused the error. Switch to the more verbose "documentation"
format so it is obvious which test ran before any unexpected crash.

.rspec

@@ -1,2 +1,3 @@
 --color
 --require spec_helper
+--format documentation