Use relative paths for the grammar input files

The file paths that are passed to antlr end up in the generated output
so using absolute paths breaks caching.

Fixes marcohu#12.

src/main/java/org/antlr/bazel/AntlrRules.java

@@ -376,7 +376,7 @@ AntlrRules grammars(String... grammars)
 
         for (String grammar : grammars)
         {
-            this.grammars.add(sandbox.resolve(grammar).toString());
+            this.grammars.add(grammar.toString());
         }
 
         return this;