update 2.6.x version to 2.6.17 for travis

It looks like Puppet 2.6 has been updated to 2.6.17.

this commit updates the 2.6.x version of Puppet used for 2.6.x
support in hopes that hash support is improved in this version.
Merge pull request #169 from bodepd/travis_2_6_17

update 2.6.x version to 2.6.17 for travis