Fix *snippets.rst duplicate inclusion by conf.py & sphinx-build sourcedir

This fixes the ```*snippets.rst``` files from being included more than once from the ```conf.py``` ```rst_epilog``` variable and the ```sphinx-build``` source directory. The following error can be seen in previous build outputs:

>snippets.rst:4: ERROR: Duplicate substitution definition name: "ProgrammersReferenceGuideofZendFramework2".
snippets.rst:6: ERROR: Duplicate substitution definition name: "IntroductiontoZendFramework".
snippets.rst:8: ERROR: Duplicate substitution definition name: "UserGuide".
snippets.rst:10: ERROR: Duplicate substitution definition name: "UserGuideIntroduction".
snippets.rst:14: ERROR: Duplicate substitution definition name: "LearningZendFramework".
snippets.rst:16: ERROR: Duplicate substitution definition name: "Migration".
snippets.rst:18: ERROR: Duplicate substitution definition name: "ZendFrameworkReference".
snippets.rst:20: ERROR: Duplicate substitution definition name: "ZendServiceReference".
snippets.rst:22: ERROR: Duplicate substitution definition name: "IndicesAndTables".
snippets.rst:24: ERROR: Duplicate substitution definition name: "ZFTool".

By excluding ```*snippets.rst``` from the source directory, it is only included once by the ```conf.py``` ```rst_epilog``` variable as intended.

You may confirm that the above errors no longer exist from the Travis-ci build details link and that the documentation still renders as expected: http://webdevelzf2-documentation.readthedocs.org/en/latest/.