Fix web_server for text_sensors

[OttoWinter]

Fixes https://github.com/esphome/issues/issues/94

Description:

Related issue (if applicable): fixes

Pull request in esphome with python changes (if applicable): esphome/esphome# Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#

Checklist:

• [ ] The code change is tested and works locally.
• [ ] The code change follows the standards

If user exposed functionality or configuration variables are added/changed:

• [ ] Documentation added/updated in esphome-docs.