rename rust-toolchain to rust-version; add note to README about usage

Author: apoelstra

.travis.yml

@@ -18,9 +18,8 @@ before_script:
   if [ "$TRAVIS_EVENT_TYPE" = cron ]; then
     RUST_TOOLCHAIN=nightly
   else
-    RUST_TOOLCHAIN=$(cat rust-toolchain)
+    RUST_TOOLCHAIN=$(cat rust-version)
   fi
-- rm rust-toolchain
 # install Rust
 - curl https://build.travis-ci.org/files/rustup-init.sh -sSf | sh -s -- -y --default-toolchain "$RUST_TOOLCHAIN"
 - export PATH=$HOME/.cargo/bin:$PATH

README.md

@@ -84,7 +84,9 @@ with Miri.
 
 ## Using Rustup To Specify a Specific Nightly
 
-To target a specific nightly, modify the above instructions as follows.
+To target a specific nightly, modify the above instructions as follows. It is recommended
+to use the nightly specified in the `rust-version` file in this repo, since that is the
+most recent nightly supported by Miri.
 
 1. Install Miri using `cargo +nightly-2018-10-15 install --all-features --path .`,
 with the date replaced as appropriate.

appveyor.yml

@@ -16,7 +16,7 @@ branches:
 install:
     # install Rust
     - set PATH=C:\Program Files\Git\mingw64\bin;C:\msys64\mingw%MSYS2_BITS%\bin;%PATH%
-    - set /p RUST_TOOLCHAIN=<rust-toolchain
+    - set /p RUST_TOOLCHAIN=<rust-version
     - curl -sSf -o rustup-init.exe https://win.rustup.rs/
     - rustup-init.exe -y --default-host %TARGET% --default-toolchain %RUST_TOOLCHAIN%
     - set PATH=%USERPROFILE%\.cargo\bin;%PATH%