Removed default check for curl, and placed it into nvm function

This fixes issue with curl not being installed before nvm is initially sourced
and then curl subsequently being installed.

removed creation of global curl function (after error detected) which
overwrites the system tool, preventing execution of later installed
curl binary

nvm.sh

@@ -10,12 +10,6 @@ if [ ! -d "$NVM_DIR" ]; then
     export NVM_DIR=$(cd $(dirname ${BASH_SOURCE[0]:-$0}); pwd)
 fi
 
-# Emulate curl with wget, if necessary
-if [ ! `which curl` ]; then
-  NOCURL='nocurl'
-  curl() { echo 'Need curl to proceed.' >&2; }
-fi
-
 # Expand a version using the version cache
 nvm_version()
 {
@@ -75,6 +69,11 @@ print_versions()
 
 nvm()
 {
+  if [ ! `which curl` ]; then
+    echo 'NVM Needs curl to proceed.' >&2;
+    return
+  fi
+
   if [ $# -lt 1 ]; then
     nvm help
     return