use http urls for maven

Upgrading to 2.2.1 causes an error on some
of our build machines:

```
* What went wrong:
Could not resolve all dependencies for configuration ':spectator-api:checkstyle'.
> Could not resolve com.puppycrawl.tools:checkstyle:5.7.
  Required by:
      com.netflix.spectator:spectator-api:0.16-SNAPSHOT
   > Could not GET 'https://repo1.maven.org/maven2/com/puppycrawl/tools/checkstyle/5.7/checkstyle-5.7.pom'.
      > peer not authenticated
```

Seems to be a known issue:

http://stackoverflow.com/questions/26675814/gradle-could-not-head-https-pom-peer-not-authenticated
Merge pull request #87 from brharrington/peer-not-authenticated

use http urls for maven