- Changes `$DEPENDENCIES` to `$INTEGRATION_DEPS` in `.travis.yml`, and
  changes conditional in `install` script to check for a non-empty value.
- Updates license docblock in new files.

PSR-7 requires a string or array of strings; if an array is provided, it
cannot be empty.

Doing so ensures that when we merge values via `withAddedHeader()`, we
retain the original values when key collissions might normally occur.

Fixes compatibility with PSR-7.

Per the PSR-7 spec, should only allow arrays, objects, or null values.

Previously, a lazy `if ($password)` check meant that a value of `0`
would be ignored; it now checks more specifically for `if (null !==
$password)`.

`MessageTraitTest` had two tests that were for behavior not supported by
PSR-7:

- `testHeaderExistsIfWithNoValues`
- `testHeaderWithNoValues`

As the integration tests demonstrate, these were not valid scenarios. As
such, the tests are removed.