Update many of the project's dependencies.
More dependency updates.
Use URI.join when appending URLs.

It is not always certain whether the permalink property of a link includes a leading slash. By using URI.join, we will ensure that we get the correct URL back, regardless of its initial format.
Remove a redundant RSpec configuration line.

`treat_symbols_as_metadata_keys_with_true_values` is now true by default. Nice!