libpqxx-3.1 should be re-builded with new C++ ABI in Xenial

[Alexander Drozdov]

Currently Xenial contains default compiller GCC 5.3/5.4 (second from updates) that uses new C++ ABI by default (https://gcc.gnu.org/gcc-5/changes.html#libstdcxx). libpqxx-3.1 package built with old ABI implementation (old compiler?). As a result: all attempts to linking code with this library fails.

Simple rebuilding and package reinstalling solves issue.

Version information:
```
$ apt-cache policy libpqxx-3.1
libpqxx-3.1:
  Installed: 1:3.1.1-0ubuntu4
  Candidate: 1:3.1.1-0ubuntu4
  Version table:
 *** 1:3.1.1-0ubuntu4 500
        500 http://archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
        100 /var/lib/dpkg/status
```

PS seems that libpqxx-4.0 affected too.