text: Title: skopt.plots in 1-dimension
  Body: I'm working on simple examples with optimization with respect to a single variable.
  Both 
  ```
  from skopt.plots import plot_evaluations
  from skopt.plots import plot_objective
  ```
  
  seem to fail if I'm only optimizing wrt a single variable
  ```
  /Users/cranmer/anaconda/lib/python3.5/site-packages/skopt/plots.py in plot_objective(result, levels, n_points, n_samples, zscale)
      305         for j in range(space.n_dims):
      306             if i == j:
  --> 307                 xi, yi = partial_dependence(space, result.models[-1], i,
      308                                             j=None,
      309                                             sample_points=rvs_transformed,
  
  IndexError: list index out of range
  ```