Fixes unexpected comma error in base router.

This error causes Dynamo to fail at compile with a stack trace similar to this:

    ** (SyntaxError) /Users/cloud/Projects/open-source/storyteller/deps/dynamo/lib/dynamo/router/base.ex:270: unexpected comma. Parentheses are required to solve ambiguity in nested calls. Syntax error before: ','
    src/elixir_parser.yrl:621: :elixir_parser.throw/3
    src/elixir_parser.yrl:377: :elixir_parser.yeccpars2_282/7
    /usr/local/Cellar/erlang/R16B/lib/erlang/lib/parsetools-2.0.9/include/yeccpre.hrl:56: :elixir_parser.yeccpars0/5
    src/elixir_translator.erl:16: :elixir_translator.forms/4
    src/elixir_translator.erl:28: :elixir_translator.forms!/4
    src/elixir_compiler.erl:24: :elixir_compiler.string/2
    src/elixir_compiler.erl:48: :elixir_compiler.file_to_path/2
    /Users/cloud/Projects/open-source/elixir/lib/elixir/lib/kernel/parallel_compiler.ex:68: Kernel.ParallelCompiler."-spawn_compilers/7-fun-0-"/3