Add specs for Rex::Exploitation::EncryptJS

Verification
-----------

- [x] Run rspec for the new specs and ensure there aren't failures:

```
$ rspec spec/lib/rex/exploitation/encryptjs_spec.rb
Rex::Exploitation::EncryptJS ....

Finished in 0.01146 seconds
4 examples, 0 failures

Randomized with seed 40990

Coverage report generated for RSpec to /Users/jvazquez/Projects/Code/metasploit-framework/coverage. 16065 / 91890 LOC (17.48%) covered.
```

- [x] Be sure which Travis is green