update safeslinger lib (closes #1434)

OpenKeychain/build.gradle

@@ -65,7 +65,7 @@ dependencies {
     compile project(':extern:spongycastle:prov')
     compile project(':extern:minidns')
     compile project(':extern:KeybaseLib:Lib')
-    compile project(':extern:safeslinger-exchange')
+    compile project(':extern:safeslinger-exchange:safeslinger-exchange')
     compile project(':extern:snackbar:lib')
 }