Update httpretty to 0.6.5

(but no further due to https://github.com/gabrielfalcao/HTTPretty/issues/114)
Don't set/replace/delete TroveboxObject attributes that start with an underscore.

Just ignore them, don't raise an exception
Merge pull request #60 from sneakypete81/httpretty_version_freeze

Update httpretty to 0.6.5
Merge pull request #61 from sneakypete81/ignore_illegal_attributes

Don't set/replace/delete TroveboxObject attributes that start with an underscore
Version bump to 0.5.1