setup: Flask-OAuthlib version too low

[eamonnmag]

Flask-OAuthlib>=0.6.0,<0.7

On the travis build, without pinning, 0.9.3 is installed since this is what is required by the oauth libs.

Requirement.parse('Flask-OAuthlib>=0.9.3'), set(['invenio-oauth2server', 'invenio-oauthclient']))

pkg_resources.ContextualVersionConflict: 
(Flask-OAuthlib 0.9.3 (/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages), 
Requirement.parse('Flask-OAuthlib<0.7,>=0.6.0'), set(['invenio-ext']))