Added self-update command

src/update.rs

@@ -0,0 +1,17 @@
+//! When wasmer self-update is executed, this is what gets executed
+use std::process::{Command, Stdio};
+use std::io;
+
+pub fn self_update() {
+    println!("Fetching latest installer");
+    let cmd = Command::new("curl").arg("https://get.wasmer.io").arg("-sSfL")
+            .stdout(Stdio::piped()).spawn().unwrap();
+
+    let mut the_process =  Command::new("sh")
+            .stdin(cmd.stdout.unwrap())
+            .stdout(Stdio::inherit())
+            .spawn()
+            .ok().expect("Failed to execute.");
+
+    the_process.wait();
+}