shorten proof of 3.11.4, close #520
remark on "merge of" syntax
use separate branches for pull requests
Automatically mark and push errata

This commit uses the `mark-errata` script to mark errata on travis.
If travis builds something which is the same as the official repo's
master, and we are building for the official repo, then we autocommit
updated errata and push to the master branch.

Someone who has push bits for this repo will need to replace my OAuth
key with one of their own.  I record the process here (adapted from
082425c2b6633dc66ebe2365b860e2ad88c5610f of HoTT/HoTT), so that if we
need to update the keys, there will be a reference.

First, generate the OAuth token that will be used, replacing
'<username>' with your username.  You must have permission to push to
HoTT/book.
```bash
$ TOKEN="$(curl -u '<username>' -d '{"scopes":["repo"],"note":"For Travis-CI Bot for HoTT/book"}' https://api.github.com/authorizations | grep -o 'token": "[^"]*' | sed s'/token": "//g')"
```
It will prompt you for your password.

Now, the hard part (at least if you don't have sudo). Hope that you
either have ruby and gem installed, or a version of ssh-keygen which
supports the `-e -m` flag combination (check with `ssh-keygen -\?`).  If
you have ruby and gem, then you can follow
http://about.travis-ci.org/docs/user/encryption-keys/, and run
```bash
$ gem install travis
$ travis encrypt -r HoTT/book "OAUTH_TOKEN=$TOKEN"
```
and update the `secure:` part of the `.travis.yml` file with the resulting
secret.  Otherwise, you can follow
http://about.travis-ci.org/docs/user/travis-pro/#How-can-I-encrypt-files-that-include-sensitive-data%3F
and run
```bash
$ \curl https://api.travis-ci.org/repos/HoTT/book/key | perl -pe 's/^{"key"://; s/^"//; s/"}$//' | sed s'/\\n/\n/g' > id_travis.pub
$ ssh-keygen -e -m PKCS8 -f id_travis.pub > id_travis.pub.pem
$ echo "OAUTH_TOKEN=$TOKEN" | openssl rsautl -encrypt -pubin -inkey id_travis.pub.pem
```
However, if `ssh-keygen` fails, complaining about `-m` being
unrecognized, then you must clone or copy
https://gist.github.com/thwarted/1024558
(https://gist.github.com/1024558.git), install the required python
dependencies (for me, pyasn1), and then run that on `id_travis.pub` to
get the `id_travis.pub.pem` file, and then run the `openssl rsautl`
commmand.  Either way, the output of that third command should be placed
in the `.travis.yml` file, replacing the existing secret.

Perhaps we should include these instructions somewhere in the
repository?
Merge pull request #547 from mikeshulman/contributing

Some updates to CONTRIBUTING.md
exercise 1.8 requires transitivity
unmarked errata for 6d0b239
Merge pull request #546 from JasonGross/auto-mark-errata

Automatically mark and push errata (almost)
add Mike's OAuth key to .travis.yml
Merge pull request #549 from mikeshulman/transitivity

exercise 1.8 requires transitivity, close #533
Try to fix the auto-errata-pusher

I think the problem was that it commited the update to the errata on a
detached HEAD, and then failed to transfer the commit when it rebased on
master.  Hence we now cherry-pick after rebase.
Make the errata marker work after merges

The old code listed all the merges between the erratum and the current
head.  By inspection of what the errata marking command does for
39dfbd8 and what 39dfbd8 actually got marked as, we want the last sha1
in the list.
Merge pull request #550 from JasonGross/fix-auto-errata-pusher

Fix auto errata pusher
Mark Errata (auto)
close #551
unmarked errata for 54d074e
Merge pull request #552 from mikeshulman/lemma6.12.3

close #551
Mark Errata (auto)
Merge pull request #535 from mikeshulman/lemma3.11.4

shorten proof of 3.11.4, close #520
remove commented old proof of 3.11.4