Fix Go version library incompatibility.
Fix Go version library incompatibility.

This commit replaces a new crypto/rsa.PrivateKey.Public() call with
&crypto/rsa.PrivateKey.PublicKey, which works on older versions of Go.

tmroeder/cloudproxy,cjpatton/cloudproxy,tmroeder/cloudproxy,William-J-Earl/cloudproxy,William-J-Earl/cloudproxy,jethrogb/cloudproxy,jlmucb/cloudproxy,William-J-Earl/cloudproxy,kevinawalsh/cloudproxy,William-J-Earl/cloudproxy,kevinawalsh/cloudproxy,jethrogb/cloudproxy,tmroeder/cloudproxy,tmroeder/cloudproxy,jethrogb/cloudproxy,jlmucb/cloudproxy,jethrogb/cloudproxy,cjpatton/cloudproxy,tmroeder/cloudproxy,cjpatton/cloudproxy,William-J-Earl/cloudproxy,jlmucb/cloudproxy,jlmucb/cloudproxy,jlmucb/cloudproxy,cjpatton/cloudproxy,cjpatton/cloudproxy,jethrogb/cloudproxy