Fix build with new Android Support Repository

If you do not specify the version, an error occurs with new version of Android Support Repository (because last version of support library only for L preview):
> Manifest merger failed : uses-sdk:minSdkVersion 14 cannot be smaller than version L declared in library com.android.support:support-v4:21.0.0-rc1
Merge pull request #46 from gildor/patch-1

Fix build with new Android Support Repository
Merge branch 'master' of github.com:VKCOM/vk-android-sdk