Don't overwrite existing NVM_DIR vars

nvm.sh

@@ -6,7 +6,9 @@
 # with much bash help from Matthew Ranney
 
 # Auto detect the NVM_DIR using magic bash 3.x stuff
-export NVM_DIR=$(dirname ${BASH_ARGV[0]})
+if [ ! -d "$NVM_DIR" ]; then
+    export NVM_DIR=$(dirname ${BASH_ARGV[0]})
+fi
 
 # Emulate curl with wget, if necessary
 if [ ! `which curl` ]; then