How can I improve this code to make it a better package?

Remove prints

It may have some debug value to you but your print calls will be the most annoying things for your future users. Few possibilities are there to remove it (I’m thinking about redirect_stdout to /dev/null for instance) but it add a lot of noise to a program. Besides you don't get such messages from other libraries: defaultdict don't say when they create the missing entry or they get the already existing one; json.loads don't print a success message; etc.

Add documentation

If you're going to release it for use by others, you’ll want to at least populate the help of your module with meaninfull informations. Add docstrings at all levels: module, class and functions. Some of your comments should already be docstrings anyway.

Use a standard folder location

I can cd anywhere on my filesystem and fire a Python console there. This will mean, each time, I’ll create a new 'json_cache' folder and won't benefit from past downloads. Make your folder in a unique, centralized, location so any use will benefit from it. The ~/.cache/ folder seems a good fit.

Use a better name

Your class cj is really, really badly named. Do not fear using the same name for a class/function than the name of the module. Look at socket.socket, pprint.pprint and others, it is worse to have a bad, simplified, name than to repeat yourself in such cases.

Other than that, I would change a few other things:

"""TODO: Module docstring"""

import os

import json
import requests


class cache_json(object):
    """Manage a JSON object through the cache.

    Download the associated resource from the provided URL
    when need be and retrieve the JSON from a cached file
    if possible.
    """

    CACHE_FOLDER = 'json_cache'

    def __init__(self, url, update=False):
        self.url = url
        self.filename = '{}.json'.format(url.translate({
            ord('/'): '=',
            ord(':'): '-',
        }))
        self.update = update
        self.make_cache_directory()

    @property
    def cache_folder(self):
        """Path to the cache folder"""
        return os.path.join(
                os.path.expanduser('~'),
                '.cache', self.CACHE_FOLDER)

    def download(self):
        """Perform the retrieval of the requested JSON data"""
        return requests.get(self.url).json()    

    def save(self, raw):
        """Save the provided raw JSON data into the cached file"""
        filename = os.path.join(self.cache_folder, self.filename)
        with open(filename, 'w') as out:
            json.dump(raw, out)

    def load(self):
        """Retrieve the saved JSON data from the cached file"""
        filename = os.path.join(self.cache_folder, self.filename)
        with open(filename) as cached:
            return json.load(cached)

    def make_cache_directory(self):
        """Create the cache directory if it doesn't exist"""
        os.makedirs(self.cache_folder, exist_ok=True)

    @property
    def file_exists(self):
        """Whether the cached file already exist"""
        filename = os.path.join(self.cache_folder, self.filename)
        return os.path.isfile(filename)

    @property
    def json(self):
        """The JSON data associated to the given URL.

        Either read from the cache of fetched from the Internet.
        """
        if not self.update and self.file_exists:
            return self.load()

        raw = self.download()
        self.save(raw)
        return raw

Note the use of property: the expected usage becomes:

repos = cachejson.cache_json(APIURL).json