Deprecate V1 Thesaurus functionality

Oxford Dictionaries is updating their API to a new version which
includes quite a few changes:
https://developer.oxforddictionaries.com/version2

They are moving the thesaurus functionality to its own endpoint
(instead of having part of the entries endpoint).

Use `Client#thesaurus` instead. Specifically use `Client#thesaurus` with
the `filter` parameter to define what you want.

For example:

```
Client.thesaurus(
  word: 'ace',
  language: 'en',
  params: { filter: 'synonyms,antonyms' }
)

\# Or for only synonyms/antonyms

Client.thesaurus(
  word: 'ace',
  language: 'en',
  params: { filter: 'synonyms' }
)

```

lib/oxford_dictionary/endpoints/entry_endpoint.rb

@@ -41,16 +41,43 @@ def entry_sentences(query, params = {})
       end
 
       def entry_antonyms(query, params = {})
+        warn '''
+        Client#entry_antonyms is DEPRECATED and will become non-functional
+        on June 30, 2019. Use Client#thesaurus instead. Reference
+        https://github.com/swcraig/oxford-dictionary/pull/13 for more
+        information. Check out OxfordDictionary::Endpoints::Thesaurus for the
+        interface to use. Specifically use it with
+        params: { fields: \'antonyms\' }
+        '''
+
         params[:end] = 'antonyms'
         entry_request(query, params)
       end
 
       def entry_synonyms(query, params = {})
+        warn '''
+        Client#entry_synonyms is DEPRECATED and will become non-functional
+        on June 30, 2019. Use Client#thesaurus instead. Reference
+        https://github.com/swcraig/oxford-dictionary/pull/13 for more
+        information. Check out OxfordDictionary::Endpoints::Thesaurus for the
+        interface to use. Specifically use it with
+        params: { fields: \'synonyms\' }
+        '''
+
         params[:end] = 'synonyms'
         entry_request(query, params)
       end
 
       def entry_antonyms_synonyms(query, params = {})
+        warn '''
+        Client#entry_antonyms_synonyms is DEPRECATED and will be non-functional
+        on June 30, 2019. Use Client#thesaurus instead. Reference
+        https://github.com/swcraig/oxford-dictionary/pull/14 for more
+        information. Check out OxfordDictionary::Endpoints::Thesaurus for the
+        interface to use. Specifically use it with
+        params: { fields: \'synonyms,antonyms\' }
+        '''
+
         params[:end] = 'synonyms;antonyms'
         entry_request(query, params)
       end