Upgrade build.gradle minSdk to 15 and remove 9oldandroids

Many forks have this commit engaged, and as for me, it would be great to cut oldish `api<15` from further support due to thousands of methods `9oldandroids` brings to the project. Also, take a notice of `repositories` section, as allProjects{} makes project a decoupled one and decreased build speed, your lib has to have it's one repo defined.