Fix `keypairs` module reference in `lib/rsa.js`

Commit 2e7134dd8a introduced breakage
and was subsequently released as v2.0.4. The breakage crashes the
application using the module as it tries to `require` a module named
`Keypairs` even though it should not be capitalized.

lib/rsa.js

@@ -13,7 +13,7 @@ try {
   /* ignore */
 }
 
-var Keypairs = require('Keypairs');
+var Keypairs = require('keypairs');
 var RSACSR = require('./rsa-csr');
 var NOBJ = {};
 var DEFAULT_BITLEN = 2048;