[CI] Check riscv32i-unknown-none-elf target

riscv-rt/.travis.yml

@@ -7,6 +7,7 @@ rust:
 
 env:
 - TARGET=x86_64-unknown-linux-gnu
+- TARGET=riscv32i-unknown-none-elf
 - TARGET=riscv32imac-unknown-none-elf
 - TARGET=riscv64imac-unknown-none-elf