Add back x86_64-unknown-freebsd to CI.

Author: autozimu

.travis.yml

@@ -14,7 +14,7 @@ env:
 matrix:
   include:
     # Linux
-    - env: TARGETS=i686-unknown-linux-musl:x86_64-unknown-linux-musl:aarch64-unknown-linux-gnu:i686-pc-windows-gnu:x86_64-pc-windows-gnu
+    - env: TARGETS=i686-unknown-linux-musl:x86_64-unknown-linux-musl:aarch64-unknown-linux-gnu:i686-pc-windows-gnu:x86_64-pc-windows-gnu:x86_64-unknown-freebsd
 
     # OSX
     - env: TARGETS=x86_64-apple-darwin