specify the minimal profile when installing rust toolchain

this is needed bc of some bug(?) in rustup not being able to install the
necessary toolchain without error using the `default` profile

see my comment in the rustup project:
rust-lang/rustup#2661 (comment)

src/rustup.rs

@@ -57,7 +57,7 @@ pub fn available_targets(toolchain: &str, verbose: bool) -> Result<AvailableTarg
 
 pub fn install_toolchain(toolchain: &str, verbose: bool) -> Result<()> {
     Command::new("rustup")
-        .args(&["toolchain", "add", toolchain])
+        .args(&["toolchain", "add", toolchain, "--profile", "minimal"])
         .run(verbose)
         .chain_err(|| format!("couldn't install toolchain `{}`", toolchain))
 }