use home-or-tmp module instead of user-home

The main point about using this instead of just falling back in code is that it depends on an `os.tmpdir()` polyfill [0], which means the tmpdir handling is the same no matter node/iojs version. This is useful as the core `os.tmpdir()` function has changed a lot between node versions.

[0]: https://github.com/sindresorhus/os-tmpdir

---

`os.tmpdir()` diff between Node 0.10.38 and iojs 2.0.2

```diff
+const trailingSlashRe = isWindows ? /[^:]\\$/
+                                  : /.\/$/;
+
 exports.tmpdir = function() {
-  return process.env.TMPDIR ||
-         process.env.TMP ||
-         process.env.TEMP ||
-         (process.platform === 'win32' ? 'c:\\windows\\temp' : '/tmp');
+  var path;
+  if (isWindows) {
+    path = process.env.TEMP ||
+           process.env.TMP ||
+           (process.env.SystemRoot || process.env.windir) + '\\temp';
+  } else {
+    path = process.env.TMPDIR ||
+           process.env.TMP ||
+           process.env.TEMP ||
+           '/tmp';
+  }
+  if (trailingSlashRe.test(path))
+    path = path.slice(0, -1);
+  return path;
 };
```

src/babel/api/register/cache.js

@@ -1,9 +1,9 @@
 import path from "path";
 import os from "os";
 import fs from "fs";
-import userHome from "user-home";
+import homeOrTmp from "home-or-tmp";
 
-const FILENAME = process.env.BABEL_CACHE_PATH || path.join(userHome || os.tmpdir(), ".babel.json");
+const FILENAME = process.env.BABEL_CACHE_PATH || path.join(homeOrTmp, ".babel.json");
 var data = {};
 
 export function save() {