Set the global node version in the site manifest

It doesn't look like the global node version gets set anywhere. I had to add this change to our site manifest to fix.

Before:
```
Last login: Wed Jun 19 11:39:04 on ttys002
nodenv: couldn't find any version specified for use
smcnabb@mac:~ $ node --version
nodenv: couldn't find any version specified for use
smcnabb@mac:~ $ nodenv version
nodenv: couldn't find any version specified for use
smcnabb@mac:~ $ set | grep NODE
NODENV_ROOT=/opt/boxen/nodenv
NODE_PATH=/opt/boxen/nodenv/versions//lib/
```

After:
```
Last login: Wed Jun 19 11:39:14 on ttys002
smcnabb@mac:~ $ node --version
v0.10.7
smcnabb@mac:~ $ nodenv version
v0.10
smcnabb@mac:~ $ set | grep NODE
NODENV_ROOT=/opt/boxen/nodenv
NODE_PATH=/opt/boxen/nodenv/versions/v0.10/lib/
```