Improve test coverage

[KingDarBoja]

Aiming to improve test coverage and also provide little improvements to the gql package.

• References Fix/Feature: Objects via params using DSL #24: Now the DSL support using mutation as stated on the referenced PR.
• Add dsl mutation test.
• Add http transport raise for status error test.
• Add coveralls dev dependency and fix Travis CI to trigger it.
• Add arg serializer test for GraphQLList and GraphQLEnum.
• Remove get_ast_value from dsl.py as this is being done by get_arg_serializer.

[KingDarBoja]

@Cito This is ready for review :D

[Cito]

See my two remarks, otherwise LGTM.