Update dependencies (nightly-2021-06-01)

[viper-admin]

• Update rustc version to nightly-2021-06-01.
• Manualy update outdated dependencies (see the list below).
• Manualy run cargo update.

List of direct outdated dependencies:

$ cargo outdated --root-deps-only --workspace

prusti-contracts-impl
================
Name         Project  Compat  Latest  Kind    Platform
----         -------  ------  ------  ----    --------
proc-macro2  1.0.26   ---     1.0.27  Normal  ---

prusti-specs
================
Name         Project  Compat  Latest  Kind    Platform
----         -------  ------  ------  ----    --------
proc-macro2  1.0.26   ---     1.0.27  Normal  ---

prusti-contracts-internal
================
Name         Project  Compat  Latest  Kind    Platform
----         -------  ------  ------  ----    --------
proc-macro2  1.0.26   ---     1.0.27  Normal  ---

prusti-server
================
Name     Project  Compat  Latest  Kind    Platform
----     -------  ------  ------  ----    --------
futures  0.1.31   ---     0.3.15  Normal  ---
reqwest  0.9.24   ---     0.11.3  Normal  ---
tokio    0.1.22   ---     1.6.1   Normal  ---
warp     0.1.23   ---     0.3.1   Normal  ---

prusti-launch
================
Name  Project  Compat  Latest  Kind    Platform
----  -------  ------  ------  ----    --------
nix   0.20.0   ---     0.21.0  Normal  cfg(unix)

test-crates
================
Name      Project  Compat  Latest  Kind    Platform
----      -------  ------  ------  ----    --------
rustwide  0.13.0   ---     0.13.1  Normal  ---

vir
================
Name         Project  Compat  Latest  Kind    Platform
----         -------  ------  ------  ----    --------
proc-macro2  1.0.26   ---     1.0.27  Normal  ---
thiserror    1.0.24   ---     1.0.25  Normal  ---

vir-gen
================
Name         Project  Compat  Latest  Kind    Platform
----         -------  ------  ------  ----    --------
proc-macro2  1.0.26   ---     1.0.27  Normal  ---

@fpoli could you take care of this?

[fpoli]

Updating to nightly-2021-06-01 is blocked by rust-lang/rust#85574, because git2 does not compile (for a good reason, it seems) and rustwide depends on it. I'll update rustc to 2021-05-20 until rustwide releases a new version (PR for that).

[fpoli]

The dependencies of prusti-server will be updated by PR #415, which is probably still blocked by rust-lang/rust#82151.