[travisci] fix a few warnings in the travisci tool
@@ -39,8 +39,10 @@ env:
- PROJECT=qemu-mips-test TOOLCHAIN=mips-elf-7.3.0-Linux-x86_64 DEBUG=0
- PROJECT=sifive-e-test TOOLCHAIN=riscv32-elf-7.3.0-Linux-x86_64 DEBUG=0
-# run in a container
-sudo: false
+# build environment is linux xenial
+os: linux
+dist: xenial
+arch: amd64
# try to cache the toolchain download
cache:
