Put cache in the correct place

Closes #90.

.gitignore

@@ -2,7 +2,8 @@
 *.pyc
 cached_func_calls/*
 site/_posts/*
-proselint/cache/*
 profile_output
 site/write/*
 tests/corpus/newyorker/*
+cache/*
+proselint/cache/*

proselint/tools.py

@@ -9,11 +9,25 @@
 import re
 
 
+def on_heroku():
+    """Figure out if we're on Heroku."""
+    if 'DYNO' in os.environ:
+        return True
+    else:
+        return False
+
+
 def memoize(f):
     """Cache results of computations on disk."""
     path_of_this_file = os.path.dirname(os.path.realpath(__file__))
-    cache_dirname = os.path.join(path_of_this_file, "cache")
 
+    # Determine the location of the cache.
+    if on_heroku():
+        cache_dirname = os.path.join(os.path.dirname(path_of_this_file), "tmp")
+    else:
+        cache_dirname = os.path.join(path_of_this_file, "cache")
+
+    # Create the cache if it does not already exist.
     if not os.path.isdir(cache_dirname):
         os.mkdir(cache_dirname)