Update per elixir 1.4 deprecations

[russmatney]

Re: #125 – Replaces Dict usage with Keyword or Map where necessary, removing distracting compile time warnings.

[russmatney]

Failing test seems to be caused by an expiring certificate:

1) test ssl connection (ConnectionTest)
     test/connection_test.exs:109
     ** (EXIT from #PID<0.1758.0>) an exception was raised:
         ** (CaseClauseError) no case clause matching: {:error, {:tls_alert, 'certificate expired'}}
             (rethinkdb) lib/rethinkdb/connection.ex:225: RethinkDB.Connection.connect/2
             (rethinkdb) lib/rethinkdb/connection.ex:215: RethinkDB.Connection.init/1
             (connection) lib/connection.ex:425: Connection.init_it/6
             (stdlib) proc_lib.erl:240: :proc_lib.init_p_do_apply/3

[hamiltop]

Hey thanks for taking a look at this. If you could update the travis config to use 1.4, and then get it passing on 1.3 I'm happy to merge this.

[russmatney]

@hamiltop Seems to be passing on OTP 19.1 but failing on 18.3 - not sure why, will look into it hopefully later this weekend

[norpan]

Should probably have both elixir 1.3 and 1.4 in travis (and 1.5?) as well the latest versions of OTP 18, 19 and 20

[norpan]

Current master seems to not work with all combinations, or maybe there are intermittent errors. Needs checking before relying on travis testing for different versions. See #146