Satisfy lintian

Fix a number of lintian reported errors on the package:
  E: php-wikidiff2: missing-dependency-on-phpapi
  W: php-wikidiff2: latest-debian-changelog-entry-changed-to-native
  E: php-wikidiff2: duplicate-conffile etc/php5/conf.d/wikidiff2.ini
  E: php-wikidiff2: helper-templates-in-copyright
  E: php-wikidiff2: copyright-contains-dh_make-todo-boilerplate
  W: php-wikidiff2: copyright-without-copyright-notice

Change-Id: I2385110488837b1839bdcf7fa9246b5943ef27a5