Bump wallabag version to 1.7.1

Wallabag 1.7.1 fixes a couple of security vulnerabilities, adds ePUB
export and more.

For a list of changes since 1.6.1b see:
- https://www.wallabag.org/blog/2014/05/29/1-7-epub-multi-users-available/
- https://www.wallabag.org/blog/2014/07/15/wallabag-1-7-1/
Merge pull request #268 from lvillani/wallabag-1.7.1

Merge pull request #268
Indicate how to make dovecot and znc password hashes in Python

Solves the chicken-or-the-egg problem of generating a dovecot and znc salted
password hash without already having dovecot and znc installed. That is, people
are installing sovereign typically do not have dovecot and znc installed and
have no way of generating the required salted password hashes.

Python 3 standard library generates SHA512-CRYPT salted password hashes by
default so it is particularly convenient for configuring dovecot:

    python3 -c 'import crypt; print(crypt.crypt('password'))'

For ZNC, the required command is slightly more verbose.
Make sure to remove 'install' directory if wallabag is already installed

Subsequent runs of wallabag's playbook result in the 'install' directory being
recreated, (after the "Clone wallabag" step) kicking wallabag's first time
setup procedure.

With this patch we automatically remove the 'install' directory if wallabag's
configuration file appears to be in place before cloning the repository.
Vagrantfile: Debian/Ubuntu + vagrant cachier multi-machine configuration

Update the Vagrantfile so that it is possible (and easier) to spin up Debian
7, Ubuntu 12.04 and Ubuntu 14.04 VMs. Although Vagrant allows spinning up all
of them simultaneously, there may be conflicts assigning host names and IP
addresses.

This Vagrantfile also enables the vagrant-cachier plugin (if installed) to
cache downloaded packages across subsequent runs of the same VM.

Running "vagrant up" will bring up only the Debian VM since it is the
officially supported Linux distributions. Ubuntu 12.04 and Ubuntu 14.04 must
be started explicitly by running either "vagrant up precise" or "vagrant up
trusty".
Note Python 3.3 or higher required for generating dovecot and znc hashes

Also makes code slightly terser.
Merge pull request #269 from lvillani/vagrantfile

Merge pull request #269
Merge pull request #271 from lvillani/wallabag-dont-reinstall-on-upgrade

Merge pull request #271
Merge pull request #270 from ariddell/feature/hashes

Merge pull request #270