Bump minimum supported rust version from 1.31 to 1.32

[ice1000]

Signed-off-by: ice1000 [email protected]

As title. According to #250 (comment)

[nrc]

Could you add a comment about this in the README please?

[ice1000]

error[E0658]: `cfg(target_vendor)` is experimental and subject to change (see issue #29718)
   --> C:\Users\travis\.cargo\registry\src\github.com-1ecc6299db9ec823\backtrace-0.3.32\src\lib.rs:147:45
    |
147 | #[cfg(all(windows, feature = "dbghelp", not(target_vendor = "uwp")))]
    |                                             ^^^^^^^^^^^^^^^^^^^^^
error[E0658]: `cfg(target_vendor)` is experimental and subject to change (see issue #29718)
   --> C:\Users\travis\.cargo\registry\src\github.com-1ecc6299db9ec823\backtrace-0.3.32\src\backtrace\mod.rs:140:59
    |
140 |     } else if #[cfg(all(windows, feature = "dbghelp", not(target_vendor = "uwp")))] {
    |                                                           ^^^^^^^^^^^^^^^^^^^^^
error[E0658]: `cfg(target_vendor)` is experimental and subject to change (see issue #29718)
   --> C:\Users\travis\.cargo\registry\src\github.com-1ecc6299db9ec823\backtrace-0.3.32\src\symbolize\mod.rs:462:52
    |
462 |                         any(unix, all(windows, not(target_vendor = "uwp"), target_env = "gnu")),
    |                                                    ^^^^^^^^^^^^^^^^^^^^^
error: aborting due to 3 previous errors

Seems that we have to bump to 1.33?

[ice1000]

PTAL @nrc @Hoverbear

[nrc]

lgtm