fs.constants.R_OK in utils.ts is not compatible with Node 4.x LTS.

[xuqingkuang]

In the code:
https://github.com/kulshekhar/ts-jest/blob/master/src/utils.ts#L29

Is using fs.constants.R_OK, but fs.constants is not exist in Node 4.

https://nodejs.org/dist/latest-v4.x/docs/api/fs.html#fs_fs_access_path_mode_callback

I'm hoping author could a add fs.constants checking, just like

const R_OK = fs.constants && fs.constants.R_OK || fs.R_OK;
try {
    fs.accessSync(filePath, R_OK);
}

[Igmat]

We have more general issue now (#36), so I'm closing this one in favor of it.