[Koha] Cannot upgrade Koha on Ubuntu: unmet dependencies

[Rubeno Fernández]

Dear all,
This is Rubén Fernández Asensio, I maintain the catalogue of the Catalan 
Esperanto Association at http://biblioteko.esperanto.cat.
I need some help with my Koha installation: for the life of me I can't upgrade 
to the most recent version.

This is the version information shown by Koha:
Koha version: 	3.18.04.000
OS version ('uname -a'): 	Linux biblioteko-esperanto-cat 3.13.0-141-generic 
#190-Ubuntu SMP Fri Jan 19 12:53:50 UTC 2018 i686 i686 i686 GNU/Linux
Perl version: 	5.018002
MySQL version: 	mysql Ver 14.14 Distrib 5.5.59, for debian-linux-gnu (i686) 
using readline 6.3
Apache version: 	
Zebra version: 	Zebra 2.0.44

My OS is Ubuntu 14.04.5 LTS.

I installed Koha from Debian packages. These are my repos at /etc/apt/
sources.list.d/koha.list and at /etc/apt/sources.list.d/koha.list.distUpgrade:
deb http://debian.koha-community.org/koha oldstable main

Whenever I try apt-get upgrade and apt-get dist-upgrade, the package koha-
common is kept back at its current version, although aptitude shows me 17.05 
is available. 

The problem seem to be dependencies. When I run apt-get -u install koha-common 
it complains that it depends on libmojolicious-perl (>= 6.0) and libswagger2-
perl (>= 0.59), which cannot be installed.

When I run apt-get -o DeBug::pkgProblemResolver=yes dist-upgrade I get this 
output:

Starting 2 pkgProblemResolver with broken count: 3
Investigating (0) libjson-validator-perl [ i386 ] < none -> 0.67+dfsg-1~koha1 
> ( perl )
Broken libjson-validator-perl:i386 Depèn on libmojolicious-perl [ i386 ] < 
none -> 6.15+dfsg-1~koha1 > ( universe/perl ) (>= 6.00)
  Considering libmojolicious-perl:i386 0 as a solution to libjson-validator-
perl:i386 0
  Holding Back libjson-validator-perl:i386 rather than change libmojolicious-
perl:i386
Investigating (0) koha-common [ i386 ] < 3.18.04 -> 17.05.08-1 > ( web )
Broken koha-common:i386 Depèn on libmojolicious-perl [ i386 ] < none -> 
6.15+dfsg-1~koha1 > ( universe/perl ) (>= 6.0)
  Considering libmojolicious-perl:i386 0 as a solution to koha-common:i386 0
  Holding Back koha-common:i386 rather than change libmojolicious-perl:i386
Investigating (0) libswagger2-perl [ i386 ] < none -> 0.77-1~koha1 > ( perl )
Broken libswagger2-perl:i386 Depèn on libjson-validator-perl [ i386 ] < none -
> 0.67+dfsg-1~koha1 > ( perl )
  Considering libjson-validator-perl:i386 0 as a solution to libswagger2-
perl:i386 -1
  Holding Back libswagger2-perl:i386 rather than change libjson-validator-
perl:i386
 Try to Re-Instate (1) koha-common:i386

All this is arcane to me and I don't know how to go on from here. I've been 
using Linux for 10 years but I'm new to Debian and Ubuntu, and find Mandriva's 
package managing much easier.
Could anybody help me, pretty please??