Update rust-toolchain to nightly-2019-05-22

[elichai]

rust-toolchain file doesn't match baidu's toolchain apache/incubator-teaclave-sgx-sdk@e0fa56e

This should also fix the annoying no_mangle warnings rust-lang/rust#60050

[AvishaiW]

Thanks! surprised I didn't get an alert on this change

[moriaab]

Thank you @elichai!

[lacabra]

The CI build is failing with the following error:

bindgen Enclave_t.h --default-enum-style=rust --rust-target=nightly --no-recursive-whitelist --use-array-pointers-in-arguments --whitelist-function ocall_.* --raw-line "#![allow(dead_code)] use enigma_types::*; use sgx_types::*;" -- -I/opt/sgxsdk/include -I/root/sgx/edl > src/auto_ffi.rs
--
126 | error: 'rustfmt' is not installed for the toolchain 'nightly-2019-05-22-x86_64-unknown-linux-gnu'
127 | To install, run `rustup component add rustfmt --toolchain nightly-2019-05-22-x86_64-unknown-linux-gnu`
128 | Failed to run rustfmt: Internal rustfmt error (non-fatal, continuing)
129 | rustfmt src/auto_ffi.rs
130 | error: 'rustfmt' is not installed for the toolchain 'nightly-2019-05-22-x86_64-unknown-linux-gnu'
131 | To install, run `rustup component add rustfmt --toolchain nightly-2019-05-22-x86_64-unknown-linux-gnu`
132 | Makefile:30: recipe for target 'bindgen' failed

I have observed this error already, and reported it on #183, and provided a fix in scrtlabs/discovery-docker-network#26, but it looks like this requires a fix in enigma-core/Dockerfile/dockerfile and push an updated docker image for enigmampc/enigma-core

Cc: @AvishaiW, @moriaab

[AvishaiW]

@lacabra there are some toolchain updates with breaking changes. in a situation that you upgrade manually, make sure you run the rustup add command as well:
rustup target add wasm32-unknown-unknown && rustup component add rustfmt, i've done it and it seems to run just fine.

[AvishaiW]

@elichai since you aren't a member, the CI won't trigger. so, unfortunately, I need to close and reopen this PR with my user. Thank you very much for the notification and contribution!