Importing GPyOpt causes matplotlib import

[apaleyes]

Because of the chain of imports put in __init__.py files, when someone tries to import GPyOpt they end up importing matplotlib, even if they have no intention of doing any plotting. This needs to be fixed.

Minimal steps to reproduce:

1. Create a fresh empty virtual env, activate it. Don't use Anaconda and such

virtualenv gpyopt-test
source gpyopt-test/bin/activate

2. Install GPyOpt only
   pip install GpyOpt
3. Import GPyOpt
   python -c "import GPyOpt"
4. Observe the error
   ModuleNotFoundError: No module named 'matplotlib'

Proper way to fix this would be to clean up imports done by default, as well as to consider if matplotlib needs to become a hard dependency