Update Appveyor config to accept builds targeting maintenance branches

[kitchoi]

This PR updates the Appveyor configuration to accept builds from maintenance branches. Without this setting, PRs targeting a maintenance branch won't be built on Appveyor.

This is such that in the future when new maintenance branches are created from master, there is no need to change this setting again.

See https://www.appveyor.com/docs/branches/#white--and-blacklisting

[rahulporuri]

LGTM

[codecov-io]

Codecov Report

Merging #751 into master will increase coverage by 0.53%.
The diff coverage is 93.54%.

@@            Coverage Diff             @@
##           master     #751      +/-   ##
==========================================
+ Coverage   40.31%   40.85%   +0.53%     
==========================================
  Files         502      506       +4     
  Lines       27652    27813     +161     
  Branches     4193     4217      +24     
==========================================
+ Hits        11148    11362     +214     
+ Misses      16010    15977      -33     
+ Partials      494      474      -20     

Impacted Files	Coverage Δ
pyface/ui/qt4/data_view/data_view_widget.py	77.60% <80.00%> (-2.25%)	⬇️
pyface/ui/qt4/data_view/data_view_item_model.py	55.46% <81.39%> (+6.15%)	⬆️
pyface/data_view/exporters/item_exporter.py	81.81% <81.81%> (ø)
pyface/data_view/abstract_data_exporter.py	100.00% <100.00%> (ø)
pyface/data_view/data_formats.py	100.00% <100.00%> (ø)
pyface/data_view/exporters/row_exporter.py	100.00% <100.00%> (ø)
pyface/data_view/i_data_view_widget.py	95.61% <100.00%> (+0.11%)	⬆️
pyface/ui/qt4/code_editor/code_widget.py	44.39% <100.00%> (+1.93%)	⬆️
pyface/ui/qt4/code_editor/find_widget.py	100.00% <100.00%> (ø)
pyface/ui/qt4/code_editor/gutters.py	52.38% <100.00%> (+1.56%)	⬆️
... and 27 more

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