jshint make target is missing a few directories



Currently it looks like we're missing a few files/folders in the `make jshint` target ([Makefile:34-35](https://github.com/mozilla-services/msisdn-gateway/blob/54f1eba43336e7b7c3cbf0411fec469b1e05ee13/Makefile#L34-L35)):

```sh
$ make jshint && echo $?
0
```

But it doesn't look like we're linting all the files.
```sh
$ jshint msisdn-gateway/ test/*.js tools/*.js
msisdn-gateway/sms/nexmo.js: line 13, col 7, Confusing use of '!'.

msisdn-gateway/storage/proxy.js: line 6, col 36, Missing semicolon.

tools/test_encryption.js: line 5, col 5, 'digitsCode' is defined but never used.

3 errors
```