add #![feature(exclusive_range_pattern)]

Getting the following compilation error when directive is not specified:

error[E0658]: exclusive range pattern syntax is experimental
  --> src/vga_buffer.rs:74:17
   |
74 |                 0x20..0x7e | b'\n' => self.write_byte(byte),
   |                 ^^^^^^^^^^
   |
   = note: for more information, see rust-lang/rust#37854
   = help: add #![feature(exclusive_range_pattern)] to the crate attributes to enable

src/main.rs

@@ -3,6 +3,7 @@
 #![feature(custom_test_frameworks)]
 #![test_runner(blog_os::test_runner)]
 #![reexport_test_harness_main = "test_main"]
+#![feature(exclusive_range_pattern)]
 
 use blog_os::println;
 use core::panic::PanicInfo;