document and enforce minimum rustc requirement of 1.49.0

[jeremyandrews]

• document and enforce minimum rustc version of 1.49.0
• fixes document and/or enforce minimum rust version #265
• goose depends on flume for communication between threads, which in turn depends on spinning_top which uses hint::spin_loop which stabilized in rustc version 1.49.0. More detail in Tracking issue for hint::spin_loop (renamed sync::atomic::spin_loop_hint) rust-lang/rust#55002.

In the event that goose is compiled with an earlier version of rustc it will fail and provide a helpful error:

   Compiling goose v0.11.1-dev (~/goose)
error: failed to run custom build command for `goose v0.11.1-dev (~/goose)`

Caused by:
  process didn't exit successfully: `~/goose/target/release/build/goose-0b1c363936c37b47/build-script-build` (exit code: 1)
  --- stderr
  goose dependency `flume` depends on `spinning_top` crate which requires rustc >= 1.49.0.
  detected rustc version: 1.48.0
  note: see issue #55002 <https:/rust-lang/rust/issues/55002> for more information

Prior to this change, compilation of goose instead failed with a more confusing error:

   Compiling spinning_top v0.2.3
error[E0658]: use of unstable library feature 'renamed_spin_loop'
  --> /home/jandrews/.cargo/registry/src/github.com-1ecc6299db9ec823/spinning_top-0.2.3/src/spinlock.rs:57:17
   |
57 |                 hint::spin_loop();
   |                 ^^^^^^^^^^^^^^^
   |
   = note: see issue #55002 <https:/rust-lang/rust/issues/55002> for more information

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.
error: could not compile `spinning_top`.

To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
error: build failed