TravisCI build fix, by adding php ext-ssh2 to the container

Author: frenck

.travis.yml

@@ -26,6 +26,9 @@ before_install:
   - npm set progress false
 
 install:
+  - sudo apt-get update -qq
+  - sudo apt-get install -y -qq libssh2-1-dev libssh2-php
+  - pecl install -f ssh2-alpha < .noninteractive
   - pip install yamllint
   - if ! shellcheck --version ; then
       curl -sSL "${SHELLCHECK_URL}" | tar --exclude 'SHA256SUMS' --strip-components=1 -C "${HOME}/bin" -xjf -;