No longer use rustfmt-preview

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index d3547db..651ad97 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -18,7 +18,7 @@
 install:
   - rustup target add thumbv7em-none-eabi
   - rustup target add riscv32imc-unknown-none-elf
-  - rustup component add rustfmt-preview
+  - rustup component add rustfmt
   - rustup component add clippy
 
 script: