travis: Use kernel.org pre-built toolchain for riscv

This updates travis configuration to use kernel.org pre-built
toolchain for riscv.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

.travis.yml

@@ -87,9 +87,8 @@ before_script:
     fi
   - if [[ "${TOOLCHAIN}" == "powerpc" ]]; then ./tools/buildman/buildman --fetch-arch powerpc; fi
   - if [[ "${TOOLCHAIN}" == "riscv" ]]; then
-        wget https://github.com/andestech/prebuilt/releases/download/20180530/riscv64-unknown-linux-gnu.tar.gz &&
-        tar -C /tmp -xf riscv64-unknown-linux-gnu.tar.gz &&
-        echo -e "\n[toolchain-prefix]\nriscv = /tmp/riscv64-unknown-linux-gnu/bin/riscv64-unknown-linux-gnu-" >> ~/.buildman;
+       ./tools/buildman/buildman --fetch-arch riscv64;
+       echo -e "\n[toolchain-alias]\nriscv = riscv64" >> ~/.buildman;
     fi
   - if [[ "${QEMU_TARGET}" != "" ]]; then
        git clone git://git.qemu.org/qemu.git /tmp/qemu;