Force all "Previous" WP version tests to be 5.3 for now

We can move back to "previous" once WP 5.4 has shipped

.travis.yml

@@ -12,7 +12,7 @@ php:
 - "7.0"
 - "7.2"
 - "7.3"
-# 7.4 can not be included here until 5.3 is the "previous" version of WP.
+- "7.4snapshot"
 
 env:
   # Global variable is re-defined in matrix-include -list
@@ -51,9 +51,6 @@ matrix:
 
   - php: "nightly"
     name: "PHP Nightly"
-  - php: "7.4snapshot"
-    env: WP_BRANCH=master SIMPLE_AND_MULTISITE=1
-    # add a new entry for "latest" WP once 5.3 has shipped. Remove snapshot once 7.4 is GA.
 
   # Code Climate code coverage.
   - php: "7.3"

tests/setup-travis.sh

@@ -38,7 +38,7 @@ latest)
 	git clone --depth=1 --branch $(php ./tests/get-wp-version.php) git://develop.git.wordpress.org/ /tmp/wordpress-latest
 	;;
 previous)
-	git clone --depth=1 --branch $(php ./tests/get-wp-version.php --previous) git://develop.git.wordpress.org/ /tmp/wordpress-previous
+	git clone --depth=1 --branch 5.3 git://develop.git.wordpress.org/ /tmp/wordpress-previous
 	;;
 esac