Add gif to gitattributes to treat as binary

[nfahlgren]

Describe your changes
To avoid conversion of line endings between Windows, macOS, and Linux systems when cloning the git repository, we specify in .gitattributes that some file types should be treated as binary and remain unmodified. This PR adds .gif file types to that list.

Type of update
Is this a: Bug fix

For the reviewer
See this page for instructions on how to review the pull request.

• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[codecov]

Codecov Report

Merging #935 (58ce00f) into master (013e5a5) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #935   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          141       141           
  Lines         6011      6011           
=========================================
  Hits          6011      6011           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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