Fixed simple typos + composer.json keywords

Hi,

this PR does 3 small things:

- fixes spelling of ```encodeNonReservedCharaceters``` to ```encodeNonReservedCharacters```
- adds keywords to the ```composer.json``` file
- fixes typo in ```UriTemplate``` class docs

No tests were added due to the nature of the changes, but the current tests still succeed.

Btw, did you think about adding PHP 5.6 to the TravisCI setup?

Kind regards,

Steffen