Fix migrations access when packaged into egg

Use pkg_resources to access migrations files. When dexbot is packaged
into egg file (normal `setup.py install` result), we cannot just access
files inside egg (zip archive), so we're using native setuptools API to
extract the files (resource_filename()). In case files are not in egg,
pkg_resources will use normal file access.

https://setuptools.readthedocs.io/en/latest/pkg_resources.html#basic-resource-access
https://setuptools.readthedocs.io/en/latest/setuptools.html#accessing-data-files-at-runtime
http://peak.telecommunity.com/DevCenter/PythonEggs#accessing-package-resources

dexbot/storage.py

@@ -1,7 +1,6 @@
 import os
 import os.path
 import sys
-import inspect
 import json
 import threading
 import queue
@@ -227,9 +226,8 @@ def __init__(self, **kwargs):
             bundle_dir = getattr(sys, '_MEIPASS', os.path.abspath(os.path.dirname(__file__)))
             migrations_dir = os.path.join(bundle_dir, 'migrations')
         else:
-            # Path to migrations, platform-independent
-            import dexbot
-            migrations_dir = os.path.join(os.path.dirname(inspect.getfile(dexbot)), 'migrations')
+            from pkg_resources import resource_filename
+            migrations_dir = resource_filename('dexbot', 'migrations')
 
         if os.path.exists(sqlite_file) and os.path.getsize(sqlite_file) > 0:
             # Run migrations on existing database