update backtrace dependency and add gimli-symbolize feature flag

[qdequele]

I have updated the backtrace dependency and add gimli-symbolize feature flag.
This feature flag seems to fix the issue #188.
I found how to fix it in the issue rust-lang/backtrace-rs#150.

[qdequele]

Friendly reminder ;) don't hesitate to tell me if I must modify something.

[Swatinem]

@jan-auer had some concerns regarding the gimli-symbolize feature, care to explain?

[jan-auer]

I'm afraid that gimli might not yet be fully ready for replacing backtrace, so I'd just wait for rust-lang/backtrace-rs#189 to play out. Apart from that, it still adds quite some weight, so I'm not sure if I'd go with this by default.

We're in the process of splitting sentry up into multiple smaller crates. One of those might be backtrace support, and I think it may be a great idea to expose that as a feature. Alternatively, one can always include backtrace = { features = "gimli-symbolize", ... } in their Cargo.toml.

[Swatinem]

Alternatively, one can always include backtrace = { features = "gimli-symbolize", ... } in their Cargo.toml.

I will close this for now, as you can just enforce this feature on all other copies of that library.