Switch to Litho snapshot releases

Summary:
This allows breaking changes in Litho to be landed.
I'm also adding some helpers to determine if Flipper itself
is currently in a release or snapshot release train.

N.B. We don't publish snapshot releases for Flipper
right now, but will in the future where this will come in
handy.

Reviewed By: jknoxville

Differential Revision: D9359035

fbshipit-source-id: d21b607eca5d57caf4cc34234f6ba3cb0d8f307d

build.gradle

@@ -15,12 +15,19 @@ plugins {
     id 'de.undercouch.download' version '3.4.3'
 }
 
+ext.isSnapshot = { VERSION_NAME.endsWith('-SNAPSHOT') }
+ext.isRelease = { ['uploadArchives', 'bintrayUpload'].any { gradle.startParameter.taskNames.contains(it) } }
+
 subprojects {
     repositories {
         google()
         mavenLocal()
         mavenCentral()
         jcenter()
+
+        if (isSnapshot()) {
+            maven { url 'https://oss.sonatype.org/content/repositories/snapshots/' }
+        }
     }
 }