Correct Nightly errors in Makefile

Makefile

@@ -3,13 +3,13 @@ PREFIX ?= /usr/local
 BUILD = target/release/exa
 
 $(BUILD):
-	@which rustc > /dev/null || { echo "exa requires Rust Nightly to compile. For installation instructions, please visit http://rust-lang.org/"; exit 1; }
+	@which rustc > /dev/null || { echo "exa requires Rust to compile. For installation instructions, please visit http://rust-lang.org/"; exit 1; }
 	cargo build --release
 
 build: $(BUILD)
 
 build-no-git:
-	@which rustc > /dev/null || { echo "exa requires Rust Nightly to compile. For installation instructions, please visit http://rust-lang.org/"; exit 1; }
+	@which rustc > /dev/null || { echo "exa requires Rust to compile. For installation instructions, please visit http://rust-lang.org/"; exit 1; }
 	cargo build --release --no-default-features
 
 INSTALL = $(PREFIX)/bin/exa