update travis config
update travis config

cache pip packages
fix wkhtmltopdf debian package download

Antiun/odoo,factorlibre/OCB,Daniel-CA/odoo,markeTIC/OCB,factorlibre/OCB,Daniel-CA/odoo,markeTIC/OCB,Antiun/odoo,spadae22/odoo,ehirt/odoo,markeTIC/OCB,spadae22/odoo,spadae22/odoo,ehirt/odoo,Antiun/odoo,Antiun/odoo,factorlibre/OCB,Daniel-CA/odoo,markeTIC/OCB,Daniel-CA/odoo,Antiun/odoo,spadae22/odoo,Antiun/odoo,spadae22/odoo,markeTIC/OCB,factorlibre/OCB,factorlibre/OCB,ehirt/odoo,ehirt/odoo,Daniel-CA/odoo,spadae22/odoo,factorlibre/OCB,Daniel-CA/odoo,ehirt/odoo,markeTIC/OCB,ehirt/odoo,ehirt/odoo,Daniel-CA/odoo,spadae22/odoo,factorlibre/OCB,Antiun/odoo,markeTIC/OCB