README should add 'sudo apt-get install python-pip' and 'sudo pip install service_identity' to package install list

To install on a Raspberry Pi (and maybe other debian distros) I needed to also do:

sudo apt-get install python-pip

I also saw the following error on first use:

$ wormhole receive
:0: UserWarning: You do not have a working installation of the service_identity module: 'No module named service_identity'.  Please install it from <https://pypi.python.org/pypi/service_identity> and make sure all of its dependencies are satisfied.  Without the service_identity module and a recent enough pyOpenSSL to support it, Twisted can perform only rudimentary TLS client hostname verification.  Many valid certificate/hostname mappings may be rejected.
Enter receive wormhole code: 4-retrieval-atlas
Receiving file (1253 bytes) into: cert.pem
ok? (y/n): y
...

Running sudo pip install service_identity resolved this issue on future runs.

[warner]

It looks like modern Twisted tests for service_identity as soon as twisted.internet.endpoints is imported, which magic-wormhole does all the time. So even though we aren’t yet using TLS, we need it installed to avoid the warning.

I guess I’ll change the twisted dependency to be twisted[tls], which will pull in service-identity (or whatever other libraries Twisted decides it needs in the future). That will pull in a bunch of new requirements (some ASN1 modules, pyopenssl, and cryptography which has binary components), which is a drag. But I guess we’ll be moving the rendezvous server connection to TLS eventually, just on general principles, so we might as well bite the dependency bullet now.

[grempe]

@SpEcHiDe I’m not sure why a bad time would cause a request to install a package. Of course I can’t go back in time to check the state the RPi was in at the moment, but I do run ntpd on that device. It currently reports time in sync, which I am pretty sure it would have been when I reported this as well.

~ $ ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
+4.53.160.75     142.66.101.13    2 u   54 1024  377   75.685   -5.710 221.987
+2.202.196.104.b 128.138.141.172  2 u  443 1024  377   93.236   -1.121  32.146
+quirk.faceprint 18.26.4.105      2 u  965 1024  377  102.262   -4.147 203.647
*dns2.untangle.c 216.218.254.202  2 u  544 1024  377   63.192   18.058  23.495

~ $ date
Wed Dec  7 00:14:30 PST 2016