Add test badges and fix build badge

Since we're moving from Travis CI to  GitHub Actions, we need to
change badges in README.rst.

 - Adds Linux test badge for fast_testing.yml workflow status;

 - Adds OS X test badge for osx_testing.yml workflow status;

 - Fixes build badge for packaging.yml workflow status.

Author: Vasyuk Vladimir

.github/workflows/fast_testing.yml

@@ -1,4 +1,4 @@
-name: fast_testing
+name: Linux testing
 
 on:
   workflow_dispatch:

.github/workflows/osx_testing.yml

@@ -1,4 +1,4 @@
-name: osx_testing
+name: OS X testing
 
 on: [push, pull_request]
 

.github/workflows/packaging.yml

@@ -1,4 +1,4 @@
-name: packaging
+name: Packaging
 
 on: [push, pull_request]
 

README.rst

@@ -2,8 +2,17 @@
                             Tarantool C client libraries
 -------------------------------------------------------------------------------
 
-.. image:: https://travis-ci.org/tarantool/tarantool-c.svg?branch=master
-    :target: https://travis-ci.org/tarantool/tarantool-c
+.. image:: https://github.com/tarantool/tarantool-c/workflows/fast_testing/badge.svg
+    :target: https://github.com/tarantool/tarantool-c/actions/workflows/fast_testing.yml?query=branch%3Amaster
+    :alt: Linux testing
+
+.. image:: https://github.com/tarantool/tarantool-c/workflows/osx_testing/badge.svg
+    :target: https://github.com/tarantool/tarantool-c/actions/workflows/osx_testing.yml?query=branch%3Amaster
+    :alt: OS X testing
+
+.. image:: https://github.com/tarantool/tarantool-c/workflows/packaging/badge.svg
+    :target: https://github.com/tarantool/tarantool-c/actions/workflows/packaging.yml?query=branch%3Amaster
+    :alt: Packaging
 
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