Add support for Poison 4

[dazuma]

Modify gax so it will run with either Poison 3 or Poison 4. Fixes #1232.

We check at compile time for Poison.Decode.decode/2 which exists in Poison 3 but was renamed to Poison.Decode.transform/2 in Poison 4, and generate different versions of the function GoogleApis.Gax.ModelBase.poison_transform/2 in response, to call the appropriate Poison function.

This also includes two Poison 4 specific fixes:

• Poison's options are supposed to be of type Map. Most calls perform automatic conversion from Keyword, but Poison.Decode.transform doesn't. So we alter our call points accordingly.
• Poison 4 has an apparent known issue (Duplicated call to Poison.Decoder.decode/2 when using Poison.decode/2 with %{as: struct} devinus/poison#191) where struct transformation happens twice on substructs (which causes failures because the transformation code doesn't properly handle the case when an already-transformed struct is given as input). We include a workaround to prevent the second call to transform if the input is already a struct.

[dazuma]

Poison 4 requires Elixir 1.6. I have a separate PR (#2890) to update the tested Elixir versions to include 1.8 and 1.9, and remove 1.4 and 1.5.