Add support for systems without supported versions of OpenSSL

[DarkKirb]

This issue is related to #88 and would prevent issues like it when openssl 1.2 comes out.

Currently, sentry-rust depends on reqwest with default feature flags, which pulls in native-tls and openssl.

This causes multiple issues:

1. On systems without openssl header packages installed, openssl-sys won't build.
2. If distributions like arch linux update to newer versions of openssl, issues like Issue with openssl 1.1.1, update sentry to use reqwest 0.9? #88 will arise
3. Systems that don't support openssl are not supported (although targeting embedded systems with non-unix OSes is probably not a goal in this project)