Install SSH extension for all tested versions of PHP

Author: DivineOmega

.travis.yml

@@ -2,7 +2,9 @@ language: php
 dist: xenial
 before_install:
   - sudo apt-get update
-  - sudo apt-get install -y php-ssh2
+  - sudo apt-get install -y php7.1-ssh2
+  - sudo apt-get install -y php7.2-ssh2
+  - sudo apt-get install -y php7.3-ssh2
 php:
   - '7.1'
   - '7.2'