Image.py and Image_df.py is being skipped in testing because libdl and libdiplus need to be set up

[mstfbl]

Describe the bug
The testing example Image_df.py fails when run. This is because libdl and libdiplus need to be set up in NimbusML.

In test_docs_example.py, Image_df.py is temporarily skipped in the testing phase, but it will need to be put back in once the bug is fixed.

        # skip for ubuntu 14 tests
        if platform.linux_distribution()[1] == 'jessie/sid':
            # libdl needs to be setup
            if name is 'Image_df.py':
                continue
        # skip for centos7 tests 
        if platform.linux_distribution()[0] == 'CentOS Linux':
            # libgdiplus needs to be setup
            if name is 'Image_df.py':
                continue

Desktop (please complete the following information):

• OS: Ubuntu 14, CentOS
• Python version: 3.7, 3.6, 3.5, 2.7
• Version [e.g. 22]

[mstfbl]

Closing issue as the issue is fixed.