Update requests to 2.18.4

[pyup-bot]

There's a new version of requests available.
You are currently using 2.18.1. I have updated it to 2.18.4

These links might come in handy: PyPI | Changelog | Homepage

Changelog

2.18.4

+++++++++++++++++++

Improvements

• Error messages for invalid headers now include the header name for easier debugging

Dependencies

• We now support idna v2.6.

2.18.3

+++++++++++++++++++

Improvements

• Running $ python -m requests.help now includes the installed version of idna.

Bugfixes

• Fixed issue where Requests would raise ConnectionError instead of
  SSLError when encountering SSL problems when using urllib3 v1.22.

2.18.2

+++++++++++++++++++

Bugfixes

• requests.help no longer fails on Python 2.6 due to the absence of
  ssl.OPENSSL_VERSION_NUMBER.

Dependencies

• We now support urllib3 v1.22.

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖

[pyup-bot]

Closing this in favor of #92