Fix version information in bundle

`./dist/tutor --version` raises an error:

Traceback (most recent call last):
  File "main", line 4, in <module>
  File "tutor/cli.py", line 21, in main
  File "click/core.py", line 764, in __call__
  File "click/core.py", line 716, in main
  File "click/core.py", line 641, in make_context
  File "click/core.py", line 1089, in parse_args
  File "click/core.py", line 940, in parse_args
  File "click/core.py", line 1477, in handle_parse_result
  File "click/core.py", line 96, in invoke_param_callback
  File "click/decorators.py", line 270, in callback
RuntimeError: Could not determine version
[1659] Failed to execute script main

To address this, we pass the version value directly to click.

This is for issue #156.

setup.py

@@ -7,9 +7,13 @@ here = os.path.abspath(os.path.dirname(__file__))
 with io.open(os.path.join(here, "README.rst"), "rt", encoding="utf8") as f:
     readme = f.read()
 
+about = {}
+with io.open(os.path.join(here, "tutor", "__about__.py"), "rt", encoding="utf-8") as f:
+    exec(f.read(), about)
+
 setup(
     name="tutor-openedx",
-    version="3.0.0",
+    version=about["__version__"],
     url="http://docs.tutor.overhang.io/",
     project_urls={
         "Documentation": "https://docs.tutor.overhang.io/",

tutor/__about__.py

@@ -0,0 +1 @@
+__version__ = "3.0.0"

tutor/cli.py

@@ -4,6 +4,7 @@ import sys
 import click
 import click_repl
 
+from .__about__ import __version__
 from .android import android
 from .config import config
 from .dev import dev
@@ -24,7 +25,7 @@ def main():
         sys.exit(1)
 
 @click.group(context_settings={'help_option_names': ['-h', '--help', 'help']})
-@click.version_option()
+@click.version_option(version=__version__)
 def cli():
     pass