Memory optimization

`regexes.yaml` contains quite a lot of duplicated strings.
By passing `freeze: true` to `YAML.load` we allow the parser to deduplicate them.

Additionally, the regexes source and flags are never used, so they
can be removed from the long lived hashes.

Before:
```
Total allocated: 3.47 MB (28214 objects)
Total retained: 1.45 MB (8422 objects)
```

After:
```
Total allocated: 3.71 MB (30218 objects)
Total retained: 1.24 MB (4583 objects)
```

It's not a big difference in size, but it cuts down in half the number
of objects than need to be marked by the GC.

lib/user_agent_parser/parser.rb

@@ -38,16 +38,20 @@ def parse(user_agent)
     private
 
     def load_patterns(path)
-      yml = YAML.load_file(path)
+      yml = YAML.load_file(path, freeze: true)
+      [
+        parse_pattern(yml['user_agent_parsers']),
+        parse_pattern(yml['os_parsers']),
+        parse_pattern(yml['device_parsers']),
+      ]
+    end
 
-      # Parse all the regexs
-      yml.each_pair do |_type, patterns|
-        patterns.each do |pattern|
-          pattern[:regex] = Regexp.new(pattern['regex'], pattern['regex_flag'] == 'i')
-        end
+    def parse_pattern(patterns)
+      patterns.map do |pattern|
+        pattern = pattern.dup
+        pattern[:regex] = Regexp.new(pattern.delete('regex'), pattern.delete('regex_flag') == 'i')
+        pattern
       end
-
-      [yml['user_agent_parsers'], yml['os_parsers'], yml['device_parsers']]
     end
 
     def parse_ua(user_agent, os = nil, device = nil)