Unable to Specify Gamma Parameter

[acharabin]

When instantiating the XGBoost estimator using a custom gamma parameter, the following error is thrown:

TypeError: 'dict_values' object is not subscriptable

[acharabin]

After digging further, the issue doesn't appear to be directly related to the gamma parameter, but rather the indirect consequence of adding certain values of a gamma parameter that may result in a tree's leaf length of 1.

The proposed code change can be found in this pull request - 0fecf47