Rearrange schemes and fix running Tests on iOS in Xcode 6.1.1.

All that was needed was to disable modules for "Tests on iOS". I ended up doing some other housekeeping as well:

* Rename the iOS targets to "HTMLReader iOS" (for the framework) and "HTMLReader iOS Static Library" to guide future usage.
* Delete the prefix header from the Tests targets.
* Set a public headers directory for the iOS Static Library target.
Update test SDKs for Travis CI.