Issue title: getPocheVersion doesn't work ? 
 Issue body: I think there is a cache system. 
In my poche, even if I delete ./cache/prod file, my poche returns beta1 as the last version ... 

 Comments: 
 Comment 0: Maybe this is related : I just installed poche (the latest version) and here is what it says :

```
your version :
latest stable version : 1.0-beta3 . a more recent stable version is available.
```

I also have some notices in my logs : 

```
PHP Notice:  Use of undefined constant POCHE_VERSION - assumed 'POCHE_VERSION' in /home/nicofrand/www/poche/inc/poche/Tools.class.php on line 242
PHP Notice:  Use of undefined constant LANG - assumed 'LANG' in /home/nicofrand/www/poche/inc/poche/Poche.class.php on line 147 
```

Maybe the undefined constant is the root of the issue ?

 Comment 1: This bug happens to me with a beta2. 

If you have these notices with beta3, it's because inc/poche/myconfig.inc.php is not created. Do you have write access on inc/poche ?

This beta3 is buggy ...

 Comment 2: Indeed, I probably do not have write access on inc/poche. The documentation did not say this was needed.

 Comment 3: It's my fault. Beta4 will fix it (I hope).

 Comment 4: Hi,
I just installed poche. 
I have the same problem : myconfig.inc.php doesn't have been created even if I have all the access on inc/poche.
So I have this warning on my poche screen : 

Notice: Use of undefined constant POCHE_VERSION - assumed 'POCHE_VERSION' in www/poche/inc/poche/Tools.class.php on line 242 

Thanks a lot :)

--------- French --------
Hello,
merci pour ton appli, c'est juste ce qu'il me fallait en ce moment :)
J'ai donc installé tout comme il fallait mais j'ai encore cette erreur (le fichier myconfig.inc.php) n'existe pas bien qu'apache ai tous les droits sur le répertoire concerné.
Merci de ton aide !

 Comment 5: @bseclier merci pour ton message. 

@bseclier @nicofrand : I created a new issue, cf #148.

The #142 is for problem with getPocheVersion().

 Comment 6: Sorry for the flood

 Comment 7: It's not a problem with #poche. my poche was hosted on a kimsufi, and I installed a firewall, which prevent poche to fetch some articles & last version. 

{'source': 'GithubIssues', 'repo_name_with_owner': 'wallabag/wallabag', 'issue_id': 18204714, 'repo_stars': 7545.0}