Fix the Maven Central badge

[marcelstoer]

The badge should actually link to Central.

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[bdemers]

Awesome! thanks @marcelstoer

[marcelstoer]

Using https://github.com/softwaremill/maven-badges instead of shields.io for the Maven badge would give us a little more convenience but I deemed my current fix good enough for most.

[lprimak]

Shouldn't this be https://central.sonatype.com/ instead?

[marcelstoer]

I have no preference, we might just as well link to https://central.sonatype.com/artifact/org.apache.shiro/shiro-core/ instead. However, I somehow feel that search.maven.org is going to be stable far longer than central.sonatype.com.

[lprimak]

Well, it's a redirect now, so I feel like sonatype.org would better. I talked to some folks at sonatype and they say "big changs" are coming, so I would say the new stuff is gonna stick around :)