Remove Redis version from Linux scripts (#56)

* Remove Redis version from Linux scripts

* Add documentation.

src/common/thirdparty/build-redis.sh

@@ -3,9 +3,14 @@
 # Cause the script to exit if a single command fails.
 set -e
 
-if [ ! -f redis-3.2.3/src/redis-server ]; then
-  wget http://download.redis.io/releases/redis-3.2.3.tar.gz
-  tar xvfz redis-3.2.3.tar.gz
-  cd redis-3.2.3
+if [ ! -f redis/src/redis-server ]; then
+  redis_vname="redis-3.2.3"
+  # This check is to make sure the tarball has been fully extracted. The only
+  # relevant bit about redis/utils/whatisdoing.sh is that it is one of the last
+  # files in the tarball.
+  if [ ! -f redis/utils/whatisdoing.sh ]; then
+    mkdir -p "./redis" && wget -O- "http://download.redis.io/releases/$redis_vname.tar.gz" | tar xvz --strip-components=1 -C "./redis"
+  fi
+  cd redis
   make
 fi