Be less restrictive about dependencies versions and solve elixir 1.4 warnings

[Nagasaki45]

Some minor changes to mix.exs that I found necessary.

[jordan0day]

D'oh, sorry, for some reason I wasn't getting any github notifications for this project. Thanks for the 1.4 fixes, but I prefer a little more caution on the versioning.

[Nagasaki45]

Hi @jordan0day, thanks for your response.

I'm relatively new to elixir. In a project I'm working on I already have jose, httpoison, and poison as dependencies, but with different versions than those specified by big_query. I proposed this commit as it solved the issue for me and I couldn't see any downside, but maybe I'm wrong.
I would like to improve this PR, maybe by completely droping the 1st commit, please let me know what would you prefer.

[jordan0day]

Mix does have a decent workaround for your type of scenario, in your mix.exs where you're using newer versions of those libraries, you can specify override: true to basically tell mix "This is the version I really want to use!" Ex:

{:jose, "1.8.1", override: true}

If you want to drop the first commit for now, I'll test the library against the latest versions of those dependencies when I get a chance and maybe bump them then. Thanks again!

[Nagasaki45]

I dropped the versioning changes commit from the PR. Thank you very much for the tip! I learned something new :-)

[gVolop]

#7 (comment)