Fixed TravisCI, install SSH php ext differently on PHP 5.6

Author: frenck

.travis.yml

@@ -28,7 +28,11 @@ before_install:
 install:
   - sudo apt-get update -qq
   - sudo apt-get install -y -qq libssh2-1-dev libssh2-php
-  - pecl install -f ssh2-alpha < .noninteractive
+  - if [ "${TRAVIS_PHP_VERSION}" == "5.6" ]; then
+      pecl install -f ssh2 < .noninteractive
+    else
+      pecl install -f ssh2-alpha < .noninteractive
+    fi
   - pip install yamllint
   - if ! shellcheck --version ; then
       curl -sSL "${SHELLCHECK_URL}" | tar --exclude 'SHA256SUMS' --strip-components=1 -C "${HOME}/bin" -xjf -;