Revert "Disable rust master toolchain build temporarily"

This reverts commit 0d89956.

Author: matthiaskrgr

.travis.yml

@@ -75,14 +75,13 @@ matrix:
     - os: windows
 
 script:
-  # uncomment once https://github.com/rust-lang/rust/issues/55376 is fixed
-  # - |
-  #     rm rust-toolchain
-  #     cargo install rustup-toolchain-install-master || echo "rustup-toolchain-install-master already installed"
-  #     RUSTC_HASH=$(git ls-remote https://github.com/rust-lang/rust.git master | awk '{print $1}')
-  #     travis_retry rustup-toolchain-install-master -f -n master $RUSTC_HASH
-  #     rustup default master
-  #     export LD_LIBRARY_PATH=$(rustc --print sysroot)/lib
+  - |
+      rm rust-toolchain
+      cargo install rustup-toolchain-install-master || echo "rustup-toolchain-install-master already installed"
+      RUSTC_HASH=$(git ls-remote https://github.com/rust-lang/rust.git master | awk '{print $1}')
+      travis_retry rustup-toolchain-install-master -f -n master $RUSTC_HASH
+      rustup default master
+      export LD_LIBRARY_PATH=$(rustc --print sysroot)/lib
   - |
     if [ -z ${INTEGRATION} ]; then
       ./ci/base-tests.sh && sleep 5