Octoprint won't update

What is the problem?
I want to update my Octoprint to v. 1.3.7 but I keep getting the same error message no matter what I try:

Updating, please wait. Collecting https://github.com/foosel/OctoPrint/archive/1.3.7.zip Downloading https://github.com/foosel/OctoPrint/archive/1.3.7.zip (4.9MB) Exception: Traceback (most recent call last): File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/basecommand.py", line 215, in main status = self.run(options, args) File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/commands/install.py", line 324, in run requirement_set.prepare_files(finder) File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/req/req_set.py", line 380, in prepare_files ignore_dependencies=self.ignore_dependencies)) File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/req/req_set.py", line 620, in _prepare_file session=self.session, hashes=hashes) File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/download.py", line 821, in unpack_url hashes=hashes File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/download.py", line 659, in unpack_http_url hashes) File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/download.py", line 882, in _download_http_url _download_url(resp, link, content_file, hashes) File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/download.py", line 605, in _download_url consume(downloaded_chunks) File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/utils/__init__.py", line 852, in consume deque(iterator, maxlen=0) File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/download.py", line 571, in written_chunks for chunk in chunks: File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/utils/ui.py", line 139, in iter for x in it: File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/download.py", line 560, in resp_read decode_content=False): File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/_vendor/requests/packages/urllib3/response.py", line 357, in stream data = self.read(amt=amt, decode_content=decode_content) File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/_vendor/requests/packages/urllib3/response.py", line 314, in read data = self._fp.read(amt) File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/_vendor/cachecontrol/filewrapper.py", line 63, in read self._close() File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/_vendor/cachecontrol/filewrapper.py", line 50, in _close self.__callback(self.__buf.getvalue()) File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/_vendor/cachecontrol/controller.py", line 275, in cache_response self.serializer.dumps(request, response, body=body), File "/home/pi/oprint/local/lib/python2.7/site-packages/pip/_vendor/cachecontrol/serialize.py", line 87, in dumps ).encode("utf8"), MemoryError You are using pip version 9.0.1, however version 9.0.3 is available. You should consider upgrading via the 'pip install --upgrade pip' command. The update did not finish successfully. Please consult the log for details.

I hope someone here can determine what the problem is, and I'd love to know how to solve it. Let me know if you need any additional information to solve this, I'll see what I can do.
Thanks in advance!