Remove explicit setFeatureCompatibilityVersion in Travis config

A fresh 3.4 and 3.6 server should already use its most recent FCV, so we need not explicitly set it.

.travis.yml

@@ -61,7 +61,6 @@ matrix:
 
 before_script:
   - mongod --version
-  - mongo --eval 'var v = db.runCommand({buildInfo:1}).versionArray; if ((v[0] == 3 && v[1] >= 4) || v[0] >= 4) db.adminCommand({setFeatureCompatibilityVersion:"3.4"});'
   - pecl install -f mongodb-${DRIVER_VERSION}
   - php --ri mongodb
   - composer install --dev --no-interaction --prefer-source