Use `command sed` for people who alias `sed`.

Fixes #493.
diff --git a/install.sh b/install.sh
index 02464fb..8faa3af 100755
--- a/install.sh
+++ b/install.sh
@@ -45,7 +45,7 @@
     curl $*
   elif nvm_has "wget"; then
     # Emulate curl with wget
-    ARGS=$(echo "$*" | sed -e 's/--progress-bar /--progress=bar /' \
+    ARGS=$(echo "$*" | command sed -e 's/--progress-bar /--progress=bar /' \
                            -e 's/-L //' \
                            -e 's/-I /--server-response /' \
                            -e 's/-s /-q /' \