travis: use custom libssh2 package

ethomson · ethomson · commit c5eb8b4a9904 · 2018-02-25T14:33:42.000Z
Use the custom libssh2 package that is a backport of libssh2 1.8.0 to
Ubuntu trusty.
diff --git a/.travis.yml b/.travis.yml
@@ -34,7 +34,7 @@ addons:
    libcurl3
    libcurl3-gnutls
    libcurl4-gnutls-dev
-   libssh2-1-dev
+   libssh2-1
    openssh-client
    openssh-server
    valgrind
