Bump Core to 4.0.6 (#215)

Bump Core as part of the next Firebase release.

Author: ryanwilson

Firebase/Core/FIROptions.m

@@ -42,7 +42,7 @@
 NSString *const kFIRLibraryVersionID =
     @"4"     // Major version (one or more digits)
     @"00"    // Minor version (exactly 2 digits)
-    @"05"    // Build number (exactly 2 digits)
+    @"06"    // Build number (exactly 2 digits)
     @"000";  // Fixed "000"
 // Plist file name.
 NSString *const kServiceInfoFileName = @"GoogleService-Info";