Build/Test tools: Don't install PHPUnit on the `travis:js` builds. Saves a couple of minutes of build time.

See #40100


git-svn-id: https://develop.svn.wordpress.org/trunk@40271 602fd350-edb4-49c9-b593-d223f7449a82

.travis.yml

@@ -65,6 +65,7 @@ before_script:
   fi
 - |
   # Install the specified version of PHPUnit depending on the PHP version:
+  if [[ "$WP_TRAVISCI" == "travis:phpunit" ]]; then
     case "$TRAVIS_PHP_VERSION" in
       7.1|7.0|hhvm|nightly)
         echo "Using PHPUnit 5.7"
@@ -83,6 +84,7 @@ before_script:
         exit 1
         ;;
     esac
+  fi
 - npm --version
 - node --version
 - nvm install 6.9.1