add `Rollback` model

[pivotal story](https://www.pivotaltracker.com/n/projects/488191/stories/64898846)
Merge pull request #154 from bitzesty/rollbacks-history

add `Rollback` model
Add sed commands for database import

The production database dump contains various entries which try to set definer
of views to be an account which does not exist in the development VM. This will
cause errors when you try to import it in development:

```
ERROR 1227 (42000) at line 7199: Access denied; you need (at least one of) the SUPER privilege(s) for this operation
```

The offending lines look like:

```
/*!50001 CREATE ALGORITHM=UNDEFINED */
/*!50013 DEFINER=`tariff_temporal`@`%` SQL SECURITY DEFINER */
```

We can safely remove those in development.

Usage:

```
sed -f path/to/prod2dev.sed production-dump.sql > development-dump.sql
```
Update airbrake
Ignore release_xxx branches in travis

We've already tagged them as good in our CI environment; it seems
wasteful to repeat the build in travis.
Merge pull request #155 from alphagov/fix-preview

Update airbrake
update measure link scheme

[pivotal story](https://www.pivotaltracker.com/n/projects/488191/stories/69339772)
Merge pull request #157 from bitzesty/measure-link-scheme

update measure link scheme
Do not show section duplicates when searching via synonyms

[pivotal story](https://www.pivotaltracker.com/n/projects/488191/stories/68999382)
Merge pull request #159 from bitzesty/dedup-synonyms-search

Do not show section duplicates when searching via synonyms
Retry downloading update if update file is missing

When Tariff runs in multiserver environment one server may have
downloaded the update and marked as pending. If other server tries to
apply it, it won't find the file. Therefore we try to redownload if file
is missing.