Advance project version to 1.3-SNAPSHOT

Set LATEST_STABLE language version and current version of stdlib to 1.3, set IS_PRE_RELEASE
Remove "EXPERIMENTAL" from 1.3 version description in tests and gradle options

compiler/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -214,7 +214,7 @@ enum class LanguageVersion(val major: Int, val minor: Int) : DescriptionAware {
             str.split(".", "-").let { if (it.size >= 2) fromVersionString("${it[0]}.${it[1]}") else null }
 
         @JvmField
-        val LATEST_STABLE = KOTLIN_1_2
+        val LATEST_STABLE = KOTLIN_1_3
     }
 }