Don't crash if response body doesn't contain valid JSON.

[the-kenny]

Uses Poison.decode instead of its throwing variant Poison.decode! and uses the with macro for graceful error handling.

Fixes #18

[the-kenny]

Note that I tried to add a few tests for the new error behavior, but got stuck as I couldn't easily spawn a mocked Ethereumex.HttpClient

[ayrat555]

@the-kenny can you bump version and add your changes to CHANGELOG?