Make code compatible with Ruby 2.7

Not including specs etc.

lib/nostr/event.rb

@@ -149,7 +149,9 @@ module Nostr
     #
     # @return [Array<String>] The event's updated list of tags
     #
-    def add_event_reference(event_id) = tags.push(['e', event_id])
+    def add_event_reference(event_id)
+      tags.push(['e', event_id])
+    end
 
     # Adds a reference to a pubkey as a 'p' tag
     #
@@ -163,7 +165,9 @@ module Nostr
     #
     # @return [Array<String>] The event's updated list of tags
     #
-    def add_pubkey_reference(pubkey) = tags.push(['p', pubkey.to_s])
+    def add_pubkey_reference(pubkey)
+      tags.push(['p', pubkey.to_s])
+    end
 
     # Signs an event with the user's private key
     #
@@ -212,13 +216,13 @@ module Nostr
     #
     def to_h
       {
-        id:,
-        pubkey:,
-        created_at:,
-        kind:,
-        tags:,
-        content:,
-        sig:
+        id: id,
+        pubkey: pubkey,
+        created_at: created_at,
+        kind: kind,
+        tags: tags,
+        content: content,
+        sig: sig
       }
     end