Fix #2183 porterstemmer causes make json to fail

Drop PorterStemmer package support.
For now, PyStemmer can accelarate stemming process of snowballstemmer.

CHANGES

@@ -3,6 +3,8 @@ Release 1.4 (in development)
 
 Incompatible changes
 --------------------
+* Drop ``PorterStemmer`` package support. Use ``PyStemmer`` instead of ``PorterStemmer``
+  to accelerate stemming.
 
 Features added
 --------------
@@ -33,6 +35,7 @@ Bugs fixed
 * C++, add support for user-defined literals.
 * #1804: Now html output wraps overflowed long-line-text in the sidebar. Thanks to
   Hassen ben tanfous.
+* #2183: Fix porterstemmer causes `make json` to fail
 
 Documentation
 -------------

sphinx/search/en.py

@@ -12,18 +12,11 @@
 from sphinx.search import SearchLanguage
 
 try:
-    # http://bitbucket.org/methane/porterstemmer/
-    from porterstemmer import Stemmer as CStemmer
-    CSTEMMER = True
-    PYSTEMMER = False
+    from Stemmer import Stemmer as PyStemmer
+    PYSTEMMER = True
 except ImportError:
-    CSTEMMER = False
-    try:
-        from Stemmer import Stemmer as PyStemmer
-        PYSTEMMER = True
-    except ImportError:
-        from sphinx.util.stemmer import PorterStemmer
-        PYSTEMMER = False
+    from sphinx.util.stemmer import PorterStemmer
+    PYSTEMMER = False
 
 english_stopwords = set("""
 a  and  are  as  at
@@ -231,11 +224,7 @@ class SearchEnglish(SearchLanguage):
     stopwords = english_stopwords
 
     def init(self, options):
-        if CSTEMMER:
-            class Stemmer(CStemmer):
-                def stem(self, word):
-                    return self(word.lower())
-        elif PYSTEMMER:
+        if PYSTEMMER:
             class Stemmer(object):
                 def __init__(self):
                     self.stemmer = PyStemmer('porter')