Move azure pipeline to stable

.azure/azure-pipelines.yml

@@ -3,13 +3,13 @@ trigger:
 
 strategy:
   matrix:
-    linux-nightly:
+    linux-stable:
       image: ubuntu-16.04
       style: 'unflagged'
-    macos-nightly:
+    macos-stable:
       image: macos-10.14
       style: 'unflagged'
-    windows-nightly:
+    windows-stable:
       image: vs2017-win2016
       style: 'unflagged'
     linux-nightly-canary:
@@ -35,11 +35,11 @@ steps:
       then
         sudo apt-get -y install libxcb-composite0-dev libx11-dev
       fi
-      curl https://sh.rustup.rs -sSf | sh -s -- -y --no-modify-path --default-toolchain `cat rust-toolchain`
+      curl https://sh.rustup.rs -sSf | sh -s -- -y --no-modify-path --default-toolchain "stable"
       export PATH=$HOME/.cargo/bin:$PATH
       rustc -Vv
       echo "##vso[task.prependpath]$HOME/.cargo/bin"
-      rustup component add rustfmt --toolchain `cat rust-toolchain`
+      rustup component add rustfmt --toolchain "stable"
     displayName: Install Rust
   - bash: RUSTFLAGS="-D warnings" cargo test --all-features
     condition: eq(variables['style'], 'unflagged')