Allow overwriting config in phive.xml with phive install

[sebastianheuer]

When I have an installed PHAR in phive.xml and I explicitly install the same PHAR with a different version constraint, the data in phive.xml should be updated accordingly.

Example:
Data in phive.xml:

<?xml version="1.0" encoding="UTF-8"?>
<phive xmlns="https://phar.io/phive">
  <phar name="phpunit" version="^5.4.0" location="./tools/phpunit" installed="5.4.8"/>
</phive>

Install phpunit with version constraint 5.2.12:

$ phive install phpunit@5.2.12
Phive 0.6.1 - Copyright (C) 2015-2016 by Arne Blankerts, Sebastian Heuer and Contributors
Downloading https://phar.phpunit.de/phive.xml [ 65.82 KB / 65.82 KB - 100% ]
Linking phpunit-5.2.12.phar to /Users/sebastian/projects/php/phive/tools/phpunit

The PHAR gets installed correctly:

$ tools/phpunit --version
PHPUnit 5.2.12 by Sebastian Bergmann and contributors.

But phive.xml still looks the same:

<?xml version="1.0" encoding="UTF-8"?>
<phive xmlns="https://phar.io/phive">
  <phar name="phpunit" version="^5.4.0" location="./tools/phpunit" installed="5.4.8"/>
</phive>