fix typo
Avoid vague "previous versions", and state TLS versions explicitly

Several places in the spec referred to "previous versions..." but were
imported from TLS 1.2's spec -- we should mention which versions
explicitly, to avoid ambiguity about whether these refer to TLS 1.2 or
TLS 1.3.
Finish/cleanup removal of RSA key exchange

This leaves ClientKeyExchange looking a little silly, with its
apparently unnecessary select{}, but that's needed to provide the
framework for ECDHE and other key exchange mechanisms.
Faster travis run
Merge pull request #49 from martinthomson/travis-speedup

Faster travis run
Merge pull request #48 from dkg/master

fix typo
Move Content Type inside encryption

Previous versions of TLS have left the Content Type in the clear
without justifying the exposure of this piece of information.

This change moves the Content Type inside the encryption to protect
its confidentiality.

For backward compatibility during the initial handshake negotiation,
we need to keep the type field at its original location on the wire
for cleartext packets.  This means redefining the NULL_NULL case to
not simply be an pseudo-cipher that implements the identity
transformation, but to explicitly state it as sending the TLSPlaintext
struct directly over the wire.