chore(build): run regression tests from travis

Run regression tests from travis after merging to master (encrypted
files are not available for pull requests from forks).
Remove environment variables from .travis.yml if favor of per-repo
configuration on travis UI.
Temporarily remove docs generation from build.
Merge pull request #217 from silvolu/regression-keys

chore(build): run regression tests from travis
feat(datastore): support setting a property indexed value

By default, property index values are set to `true`, without allowing
the user to specify an override. Now, when a user passes in an array
to `dataset.save`, they will have the option of setting `true` or
`false`.

Example:

dataset.save({
  key: dataset.key('Company'),
  data: [
    name: 'propertyName',
    value: 'any value type',
    indexed: false
  ]
}, function(err, keys) {}).

Resolves: #208
Related: http://goo.gl/tKVvhP