Source the nvm.sh file

Fixes #775. After everything is installed, instead of restarting the terminal, just load the nvm.sh file.
diff --git a/install.sh b/install.sh
index 2ef5f8e..16d8fe1 100755
--- a/install.sh
+++ b/install.sh
@@ -227,8 +227,9 @@
   fi
 
   nvm_check_global_modules
-
-  echo "=> Close and reopen your terminal to start using nvm"
+  
+  . $NVM_DIR/nvm.sh
+  echo "=> You can now start using nvm"
   nvm_reset
 }