Automatically generate/update CHANGES.rst on Release commits to master.

[deepbrook]

Sample output:

RELEASE 0.4.0
=============
Fixes
-----
- #192  Improve PW Handling
        Closes the following issues:
        - #184 
        - #187 
        - #189 

- #238  Cross-Check Custom Token and UDC Values
        

- #242  Skip CI instead of fail on invalid commit message.
        Refactor commit message validation

- #246  Install raiden from pypi in Dockerfile.
        

- #248  Limit the included node logs of pack-logs command.
        

Hotfixes
--------
- #233  Fix BumpVersion Script.
        

- #233  Fix Bumpversion.
        

- #233  Fix branch version sync .
        Don't split commit message.

- #233  Fix branch version sync .
        Explicitly checkout files, instead of merge.

- #233  Fix branch version sync.
        Dont call `git git`, git.

- #233  Pull dev before updating its version.
        

- #255  Log Exceptions in ScenarioRunner.__init__

Implements #257

[christianbrb]

Fixes #257 ❓ 😄

[deepbrook]

Fixes #257 question smile

hopefully :D

[codecov]

Codecov Report

Merging #261 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #261   +/-   ##
=======================================
  Coverage   58.54%   58.54%           
=======================================
  Files          64       64           
  Lines        3141     3141           
=======================================
  Hits         1839     1839           
  Misses       1302     1302

---

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