Use exact version on force-lowest

Author: Nyholm

.travis.yml

@@ -36,9 +36,9 @@ matrix:
           # Test LTS versions. This makes sure we do not use Symfony packages with version greater
           # than 2 or 3 respectively.
         - php: 7.2
-          env: DEPENDENCIES="symfony/lts:^2 symfony/force-lowest:^2.8 zendframework/zend-diactoros:^1.4.1"
+          env: DEPENDENCIES="symfony/lts:^2 symfony/force-lowest:=2.8 zendframework/zend-diactoros:^1.4.1"
         - php: 7.2
-          env: DEPENDENCIES="symfony/lts:^3 symfony/force-lowest:^3.4 zendframework/zend-diactoros:^1.4.1"
+          env: DEPENDENCIES="symfony/lts:^3 symfony/force-lowest:=3.4 zendframework/zend-diactoros:^1.4.1"
 
           # Latest commit to master
         - php: 7.2