Yajl and Gson libraries should be required in lowercase

[nukeproof]

In OandaAPI::Streaming::JsonParser#try_adapter, line 94 attempts to load an adapter library.

require REQUIREMENT_MAP.fetch sym

The values in REQUIREMENT_MAP should be defined in lowercase to correctly require the libraries in line 94. Without this change, the libraries fail to be required under *nix.
change:

      REQUIREMENT_MAP = {
        gson: "Gson",
        yajl: "Yajl"
      }

to:

      REQUIREMENT_MAP = {
        gson: "gson",
        yajl: "yajl"
      }