Continuing to fix more unit tests.

-  Updated `test_specified_cv_folds`
   - Modified `make_cv_folds_data()` to use `make_classification` and generate a `FeatureSet` directly.
   - Modified `test_specified_cv_folds()` to run the tests without using config files and with and without feature hashing both.
-  Updated `test_sparse_predict`
   - Removed a few other functions that were no longer required.
   - Modified `make_sparse_data` to use `make_classification` from scikit-learn and return `FeatureSets`.
   - Modified `test_sparse_predict` to use a check function that can run tests with and without feature hashing.
-  Updated `test_scaling`
   - Create `FeatureSet` objects directly
   - Make tests deterministic with both feature hashing and scaling
-  Made sure checking large feature values in `learner.py` happens after scaling
   - Split `_check_input()` into two separate functions: `_check_input_formatting()` and `_check_max_feature_value()`. The first is called where `_check_input()` was originally called and the second is called after scaling happens inside `train()`.
- Fixed bug with default value of `train_test_ratio`
- Used `NDJWriter` instead of deprecated `write_feature_file` where we were specifically writing `.jsonlines` files.
- Collapsed all `specified_cv_folds` tests into one function
    - Modified `make_cv_folds_data` to use fewer examples otherwise the test takes too long when using `RBFSampler`.
    - Collapsed all `specified_cv_folds` tests with/without feature-hashing and with/without sampling into a single function.
I don't know if you've ever seen one of these tests fail, but it is really uninformative when it does because we're just asserting lambda functions here.

The output looks something like this:

```py
In [4]: foo = lambda x: x < 7

In [5]: assert foo(8)
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-5-39f975749de2> in <module>()
----> 1 assert foo(8)

AssertionError: 
```

It'd be a lot nicer if we replaced all of those lambda functions assertions with calls to `sklearn.utils.testing.assert_greater` and `sklearn.utils.testing.assert_less`.

```py
In [14]: from sklearn.utils.testing import assert_less

In [15]: assert_less(8, 7)
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-15-4ccb966f878c> in <module>()
----> 1 assert_less(8, 7)

/Users/dblanchard/anaconda/envs/skll_dev/lib/python3.4/unittest/case.py in assertLess(self, a, b, msg)
   1174         if not a < b:
   1175             standardMsg = '%s not less than %s' % (safe_repr(a), safe_repr(b))
-> 1176             self.fail(self._formatMessage(msg, standardMsg))
   1177 
   1178     def assertLessEqual(self, a, b, msg=None):

/Users/dblanchard/anaconda/envs/skll_dev/lib/python3.4/unittest/case.py in fail(self, msg)
    640     def fail(self, msg=None):
    641         """Fail immediately, with the given message."""
--> 642         raise self.failureException(msg)
    643 
    644     def assertFalse(self, expr, msg=None):

AssertionError: 8 not less than 7
```