Adding compatibility with Node v0.8.x's os.tmpDir()

Running 'tldr' with Node v0.8.21 reports: 
```
.../tldr/lib/cache.js:8
var FOLDER = path.join(os.tmpdir(), 'tldr-cache');
                          ^
TypeError: Object #<Object> has no method 'tmpdir'
```
This commit adds a check to call either tmpDir() or tmpdir() [renamed in Node v0.10.x].

tldr runs and passes tests with Node v0.8.21 with this commit.