Provide more guidance for understanding examples, update .travis.yml.

This commit adds an `example/README.md` file with a little bit of
guidance for running the examples themselves. This is helpful because
the examples all rely on the presence of a `phpseclib` installation
available to the PHP interpreter, and while there is a `composer.json`
file to this effect, none of the examples included the Composer
`autoload.php` file.

This commit makes no modifications to the example code itself, but does
`include_once()` the Composer autoload script so that `phpseclib` loads
and avoids causing a fatal error when a new user attempts to run the
examples to learn how to use the library.

This commit also updates the Travis `before_script` build script, dropping
the `--dev` argument to the `composer install` command. Current versions
of Composer emit a deprecation notice when `--dev` is passed.

examples/clearsign.php

@@ -1,5 +1,6 @@
 <?php
 
+@include_once dirname(__FILE__).'/../vendor/autoload.php';
 require_once dirname(__FILE__).'/../lib/openpgp.php';
 require_once dirname(__FILE__).'/../lib/openpgp_crypt_rsa.php';
 
@@ -27,5 +28,3 @@ echo "-----BEGIN PGP SIGNED MESSAGE-----\nHash: SHA256\n\n";
 // trailing whitespace to lines.
 echo preg_replace("/^-/", "- -",  $packets[0]->data)."\n";
 echo OpenPGP::enarmor($packets[1][0]->to_bytes(), "PGP SIGNATURE");
-
-?>