Use https:// link to Maven Central

(This url is just for browsing, but still we should try to use https:// where possible)
Merge pull request #45 from msgilligan/groovy-1.8

Use https:// link to Maven Central
Change Snapshot Repository to use https:// URL
Merge branch 'groovy-1.8' into groovy-1.9
Merge branch 'groovy-1.9' into groovy-2.3