Min node version required is 6.3

[aandis]

Reticketed from #457 (comment)

Expected Behavior

Login should work after upgrade if you logout and log back in.

Actual Behavior

$ clasp login
Warning: You seem to already be logged in *globally*. You have a ~/.clasprc.json

$ clasp logout
$ clasp login
Logging in globally...
Authorize clasp by visiting this url:
https://accounts.google.com/o/oauth2/v2/auth?...

Authorization successful.

/usr/local/lib/node_modules/@google/clasp/node_modules/dotf/index.js:39
          fs.chmodSync(fullpath, fs.constants.S_IRUSR | fs.constants.S_IWUSR);
                                             ^

TypeError: Cannot read property 'S_IRUSR' of undefined
    at jsonfile.writeFile (/usr/local/lib/node_modules/@google/clasp/node_modules/dotf/index.js:39:46)
    at /usr/local/lib/node_modules/@google/clasp/node_modules/graceful-fs/graceful-fs.js:111:16
    at /usr/local/lib/node_modules/@google/clasp/node_modules/graceful-fs/graceful-fs.js:45:10
    at FSReqWrap.oncomplete (fs.js:117:15)

Steps to Reproduce the Problem

This only occurs on node <= 6.2.
On login, you see the above error.

Cause

On login, dotf tries to access fs.constants.S_IRUSR which is only available in 6.3 and above. Whereas README states min node version required as 6.0.0.

Possible fixes

1. Bump the min version requirement in README.
2. Directly specify the mode as an octal value 0o600 in dotf instead of relying on the constant. From docs

An easier method of constructing the mode is to use a sequence of three octal digits.

Specifications

• Node version (node -v): 6.2.2
• Version (clasp -v): 1.7.0
• OS (Mac/Linux/Windows): Linux/Ubuntu 14.04.5 LTS

[grant]

OK. Do you want to upgrade the guide to state 6.3 is required?

[aandis]

@grant #511