Add mcrypt extension to require-dev in composer.json

If you run the test suite without the `mcrypt` extension installed you get the following failure:
```bash
$> ./vendor/bin/phpunit 
PHPUnit 4.0.14 by Sebastian Bergmann.

Configuration read from /home/mbfisher/git-projects/phpseclib/phpunit.xml.dist

...............................................................  63 / 217 ( 29%)
................................................SSSSSSSSSSSSSSS 126 / 217 ( 58%)
SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS 189 / 217 ( 87%)
SSSSSSSSSS.SSS.SS.SSF.....

Time: 237 ms, Memory: 14.00Mb

There was 1 failure:

1) Net_SSH2Test::testGenerateIdentifier with data set #5 ('SSH-2.0-phpseclib_0.3 (mcrypt, bcmath)', array('mcrypt', 'bcmath'))
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'SSH-2.0-phpseclib_0.3 (mcrypt, bcmath)'
+'SSH-2.0-phpseclib_0.3 (bcmath)'

/home/mbfisher/git-projects/phpseclib/tests/Net/SSH2Test.php:70
                                                      
FAILURES!                                             
Tests: 128, Assertions: 151, Failures: 1, Skipped: 95.
```
This commit adds `ext-mcrypt` to the `require-dev` section of `composer.json`, so that composer prompts you to install it.

Could this be the case for other extensions I happen to have installed?

M