Remove deprecation warning ('cachedir' parameter has been deprecated)

Warning was:

  /localhome/seitz_local/projects/pystencils/pystencils/cache.py:15: DeprecationWarning: The 'cachedir' parameter has been deprecated in
version 0.12 and will be removed in version 0.14.
  You provided "cachedir='/localhome/seitz_local/.cache/pystencils'", use "location='/localhome/seitz_local/.cache/pystencils'" instead.
    disk_cache = Memory(cachedir=cache_dir, verbose=False).cache

pystencils/cache.py

@@ -12,7 +12,7 @@ try:
         cache_dir = os.environ['PYSTENCILS_CACHE_DIR']
     else:
         cache_dir = user_cache_dir('pystencils')
-    disk_cache = Memory(cachedir=cache_dir, verbose=False).cache
+    disk_cache = Memory(cache_dir, verbose=False).cache
     disk_cache_no_fallback = disk_cache
 except ImportError:
     # fallback to in-memory caching if joblib is not available