tpot freezing jupyter notebook

Hi,

When training a regressor with n_jobs=-1 the process freezes after a minute. All resources go to 0 and the algorithm never stops.

## Process to reproduce the issue

```python
import sklearn
import numpy as np
import tpot

X_train = np.random.random((1000,10))
y_train = np.random.random(1000)

def RMSLE(p,a):
return np.sqrt(np.mean( (np.log(p+1) - np.log(a+1))**2 ))

rmsle_score = sklearn.metrics.make_scorer(RMSLE,greater_is_better=False)

reg1 = tpot.TPOTRegressor(verbosity=1,
n_jobs=-1,
scoring= rmsle_score,
cv=10,
max_time_mins=3)

reg1.fit(X_train, y_train)
```
I get the following

```
/Users/davidbuchacagmail.com/anaconda3/lib/python3.6/site-packages/sklearn/cross_validation.py:41: DeprecationWarning: This module was deprecated in version 0.18 in favor of the model_selection module into which all the refactored classes and functions are moved. Also note that the interface of the new CV iterators are different from that of this module. This module will be removed in 0.20.
"This module will be removed in 0.20.", DeprecationWarning)
Optimization Progress: 100%|██████████| 200/200 [01:05<00:00, 1.20pipeline/s]
Generation 1 - Current best internal CV score: 0.19893855301909574
Optimization Progress: 100%|██████████| 300/300 [01:31<00:00, 1.37pipeline/s]
Generation 2 - Current best internal CV score: 0.19893855301909574
Optimization Progress: 100%|██████████| 400/400 [01:58<00:00, 1.46pipeline/s]
Generation 3 - Current best internal CV score: 0.19893855301909574
Optimization Progress: 80%|████████ | 401/500 [02:10<00:47, 2.07pipeline/s]
```

And the process freezes even though `max_time_mins=3`.

```python
tpot.__version__
'0.9.1'
sklearn.__version__
'0.19.1'
```