reword CLA sentence in README.md

Summary: slight change to CLA sentence so ptarjan can test his external reviews hook
Closes #3137
Created from: https://github.com/facebook/hhvm/pull/3137

Reviewed By: @ptarjan

Differential Revision: D1423793
Fix for Redis EVAL / EVALSHA

Summary: phpredis casts nil responses to bool(false), whereas HHVM returns NULL for both
false and nil return values. For example:

    <?php
    $redis = new Redis();
    $redis->connect( '127.0.0.1' );
    var_dump( $redis->evaluate( 'return' ) );
    var_dump( $redis->evaluate( 'return false' ) );

With Zend / phpredis, the output is:

    bool(false)
    bool(false)

With HHVM, it's:

    NULL
    NULL

This patch adds a fix and a test.
Closes #3127

Reviewed By: ptarjan, bsimmers

Differential Revision: D1422528

Pulled By: @fredemmott