Use not freezed dependencies

[anton44eg]

Now ethjsonrpc strictly requires requests==2.9.1. I may cause problems for some projects. Do someone test it with older/newer requests?

[maurycyp]

@anton44eg I've tested with older versions of requests but not yet 2.10.0 . I know you're supposed to be less strict about versions (e.g. requests>=2) but in my experience I've been bitten by so many problems caused by dependency version changes that I prefer to pin the exact version.

I plan to give the ethjsonrpc's libraries another version bump in the near future.

[anton44eg]

Thanks!