Add „Tested up to“ and „Required PHP“, update tags

[krafit]

Fixes a couple of issues, described in #19.

I added the required information regarding the minimum supported WP and PHP version and tested WP version.
Regarding those values: 4.4 is the oldest version I was able to test Popper with, might work on older versions as well, I didn't deem it important enough to further inspect this, since the market share <4.9 is rather small.
Similar to a recent discussion regarding Antispam Bees min PHP version, 5.6 seems like the sensible option here.

The two removed tags are considered as deprecated.

[Zodiac1978]

I have checked with https://wpseek.com/pluginfilecheck/ and the Requires at least: 4.4 needs to be changed:

These two functions are only available since 4.5
get_custom_logoused in template-tags.php
has_custom_logoused in template-tags.php

But there is a check against them:

/* Test if WordPress version and whether a logo has been defined */
function popper_custom_logo() {
	if ( function_exists( 'the_custom_logo' ) && has_custom_logo() ) {
		return get_custom_logo();
	} else {
		return false;
	}
}

The next required version according to the pluginfilecheck is 4.1.