Fix Mac OS X issue in fetch-configlet script

As @jtigger kindly pointed out in https://github.com/exercism/x-api/issues/59
the fetch-configlet script doesn't compile properly with the default bash that
ships on macs.

This copies his fix from the Java track:
exercism/java@a5c9900

bin/fetch-configlet

@@ -4,25 +4,25 @@ LATEST=https://github.com/exercism/configlet/releases/latest
 
 OS=$(
 case $(uname) in
-    Darwin*)
+    (Darwin*)
         echo "mac";;
-    Linux*)
+    (Linux*)
         echo "linux";;
-    Windows*)
+    (Windows*)
         echo "windows";;
-    *)
+    (*)
         echo "linux";;
 esac)
 
 ARCH=$(
 case $(uname -m) in
-    *64*)
+    (*64*)
         echo 64bit;;
-    *686*)
+    (*686*)
         echo 32bit;;
-    *386*)
+    (*386*)
         echo 32bit;;
-    *)
+    (*)
         echo 64bit;;
 esac)