docs: Replace build badge from Travis to Actions

[a-ungurianu]

This is to reflect the updated CI pipeline used for the repo

Why is this needed?

Badge is out of date, and it currently shows a misleading Build: Failing status

Proposed Changes

• update badge to get status from latest testing workflow on master

Does this PR introduce any breaking change?

Nope

[codecov-commenter]

Codecov Report

Base: 94.45% // Head: 94.41% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (4a5a725) compared to base (a192b2e).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #675      +/-   ##
==========================================
- Coverage   94.45%   94.41%   -0.04%     
==========================================
  Files          57       57              
  Lines        8399     8399              
==========================================
- Hits         7933     7930       -3     
- Misses        466      469       +3     

Impacted Files	Coverage Δ
kazoo/tests/test_eventlet_handler.py	88.52% <0.00%> (-1.10%)	⬇️
kazoo/handlers/gevent.py	93.54% <0.00%> (-1.08%)	⬇️
kazoo/recipe/barrier.py	98.96% <0.00%> (-1.04%)	⬇️
kazoo/handlers/eventlet.py	99.01% <0.00%> (-0.99%)	⬇️
kazoo/recipe/queue.py	97.45% <0.00%> (-0.64%)	⬇️
kazoo/client.py	98.25% <0.00%> (-0.16%)	⬇️
kazoo/tests/test_client.py	98.90% <0.00%> (+0.09%)	⬆️
kazoo/protocol/connection.py	96.70% <0.00%> (+0.20%)	⬆️
kazoo/tests/test_gevent_handler.py	80.98% <0.00%> (+0.70%)	⬆️
kazoo/testing/harness.py	98.34% <0.00%> (+0.82%)	⬆️

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[StephenSorriaux]

Thank you! I was willing to do it but, for some reasons, kept forgetting about it.