Add phpunit.xml.dist to XML filenames

PHPUnit (a popular unit testing tool for PHP) uses `phpunit.xml`
for its configuration.

However it would use `phpunit.xml.dist` as well if `phpunit.xml`
is not available.

The reason is to track `phpunit.xml.dist` in your repo.
And to ignore `phpunit.xml`.
By default everyone (including a CI) would use `phpunit.xml.dist`
except you add `phpunit.xml` locally.

`phpunit.xml.dist` has the same XML structure as `phpunit.xml`.
So it should be detected as XML by linguist.

Example: https://github.com/erusev/parsedown/blob/master/phpunit.xml.dist

I don't know why linguist is not detecting this file as XML since
it starts with `<?xml`. Perhaps it is another issue.
Merge pull request #770 from hkdobrev/phpunit.xml.dist

Add phpunit.xml.dist to XML filenames